Just thought I'd respond to this - let you know where I'm coming from. Apologies for the length - I just cut and pasted into word and it's over 2500 words. I'm like a pringle - once I start I just can't stop.
First though, my usual caveats. As I think I've said before - I'm not into bodybuilding (the topic of which really covers your goals) - my attention is really on developing strength and conditioning; a quote I like is 'I prefer to be strong than look strong'. I have read a lot less about lifting recreationally - by which I mean lifting for non-competitive purposes, not meant in a derogatory tone. However, I do think there is some crossover between training for these differing reasons - after all, you can get stronger without getting bigger, but you can't get bigger without getting stronger. Unfortunately, there is plenty of conflicting information around - much of it hyperbole, and defended vociferously - but that stands for every side of the fence. I think the underlying message behind everything I write here is simple - you have to find what works for yourself. There are some immutable axioms of training - the Specific Adaptations to Imposed Demands principle springs immediately to mind - but each person is unique - I don't think there is one all-encompassing style of training. Remember - everything works, but nothing works forever. I wrote some more about this in 2 replies in this thread.
Let's start with the pseudo-scientifc reason for why I believe you can train more frequently. There is an obvious relationship between the demands imposed on your body - by training, but also by everything else going on in your life - and between recovery. It doesn't matter what model of training stimulus / response you believe in, building muscle is about getting the balance correct between breaking the muscle down, and building it back up again. Modern bodybuilding, as evangelicised in Flex, Muscle and Fitness, and others of its ilk, takes the approach of attacking the muscle as hard as possible, and then waiting a long time for it to recover - hence taking a single day to hit a muscle from different angles (which is an interesting topic by itself) with as much volume as you can take, often working to failure multiple times within the same session, and then taking a long time to recover. The problem I have with this is that I see the recovery time versus effort graph as being exponential, and not linear - that is, the closer you work to failure (repeatedly), the greater the recovery time becomes. I don't believe that the response versus effort graph follows the same exponential pattern - it has a shallower gradient. Therefore, by backing off on the total effort expended, recovery times can be reduced by a greater extent than beneficial effect - allowing more frequent training, each individually having less stimulus than a single bodybuilding style workout, but a greater density of training being allowed - the total effect being greater using this model. I dealt with this a lot in drug-development - commonly you are trying to optimise between two sigmoid curves, one for efficicy and one for toxicity. The trick with this is maximising efficacy whilst minimising tox - well, this is the same, only maximising gains whilst minimising effects on recovery.
Where do I get this opinion from? Well - I've no hard data. This comes from observation and reading. A problem with observation is that it's predominantly done on a sample size of one - me. A problem with research is that most of it is done with novices to training, who are going to respond to any stimulus. There is good research out there; I particularly recommend Zatsiorsky's book (the link to which is somewhere in one of my responses in this thread again). In addition, hanging out at several websites helps - and by this I mean a variety of ideologies, not just those who happen to agree with myself. But at the end of the day you're on your own.
Another example relating to training more frequently is comparison to other sports. Take a gymnast for example. They will come in and do some training - lets say they do a hard session on the rings. What do they do the next day? Taking the bodybuilding mentality - they can't do anything that involves their upper body, as they worked 'chest and shoulders' the previous day. The footballer or rugby player can't go and run around one day, because they ran around the previous day. No other sport puts such an arbitrary limit on the lack of frequency of training. And take a typical bodybuilding split : day 1 : chest, day 2 : back, day 3: legs, day 4 : shoulders, day 5 : arms (or whatever). Are we supposed to infer that recovery of the gluteals (for example), the largest muscles in the body, take exactly the same time to recover as the biceps (one of the smallest)? Both are being trained with the same frequency - surely this is the inference we should make. Although, in response to this, I have seen bodybuilding programs suggesting alternating frequencies depending upon muscle size and recovery rates (and even traditional bodybuilding recognises this, with ideals such as 'it's OK to train calves more than once a week'). The rest of my evidence is anecdotal. I have seen several folks increase frequency of training and have good results. And in the field of sports performance, there are plenty of people who train the same muscle groups (whatever that means) multiple times weekly - track and field, or football for example; russian powerlifters (google for Sheiko), Bill Starr routines, pretty much all olympic weightlifters, many powerlifters (google for Westside powerlifting - these guys train hamstrings, abs, triceps, upper back as much as daily at certain times), etc etc. There are obviously some important caveats : weightlifters don't have to deal with eccentrics, for example (which is probably responsible for DOMS), but still manage to squat several times weekly. I have seen written (by eminent sports research scientists, no less), that the lack of frequency of training in modern bodybuilding is more to do with lack of conditioning. This level of conditioning needs to be worked towards - but is attainable by your average joe.
Another important difference is that many of these paradigms recognise that training does not mean going to the gym and kiling yourself. Weights can be used for a variety of reasons. You can use it to stimulate force production (submax reps done as quickly as possible produce concentric forces much higher than near-max efforts). They can be used for recovery - say you squat 450-500 for a few hard reps on Monday. Then wednesday you could do 135 for a couple of sets of 12 - this helps to flush blood and nutrients through the same muscles, helping recovery (and this does have scientific rationale). You may find myth number 3 an interesting comment on this.
I personally believe that building muscle is about training with a relatively high volume at a relatively high intensity - ie 8 sets of 3, or 5 sets of 5 at 80% max. You'd be unlikely to reach failure with such sets - although you'd have to push pretty hard - but that's a helluva-total tonnage with a relatively high intensity. For example, one of Bill Starr's (you really should read up about who he is) recommendations for increasing volume over time is to initially add a back off-set