Regardless, claims of 20-30% and 8-10x are equally misinformed. A "typical" high-end (not enthusiast) Desktop is probably 50-100 percent faster in CPU terms and 200-400 percent faster in GPU terms. That's a pretty reasonable observation, I think. So, even if you're in the position of working on a game that targets the highest-end desktop PCs, a typical higher-end laptop still probably falls into the band of hardware you want to support, albeit at some "sane" resolution like 1080p. In fact, its quite reasonable to take the view that all that extra GPU power on a desktop is usually spent simply pushing more pixels, rather than spending 2-4x the compute resources on the same number of pixels, so the CPU disparity probably is the primary difference.
Of course, if you're a programmer building a huge application, you want as many of the fastest cores that you can afford, and if you're a 3D Modeler, artist, or Engine developer you want as much GPU as you can get too, and you can get more of those things in a desktop.
I'm in the exact opposite position as Samoth -- My 6 month old laptop is rather easily 2 or 3 times faster than my 3-year-old Overclocked desktop in every respect but graphics. The Quadro K2000M (Kepler-based, 384 cores) doesn't have nearly the same throughput as the Radeon 6990 in my desktop -- but the Quadro has the same hardware capabilities, and enough throughput to run games like borderlands at 1920x1200, with high settings at greater than 60 fps. If you choose well, a laptop won't stop you from doing anything at all, you'll just reach the finish line a little more slowly -- and without breaking the bank.
Comparing any "fat core" like an i-Series Processor, Core-2-duo, or even the ultra-low-end Pentium and Celeron-branded parts is unfair -- You're talking a clock-speed advantage of 1.5x minimum and 4x maxium, out-of-order execution, 2-3x more instructions in-flight, more cache, more memory bandwidth, wider SIMD. Architecturally the Atom mixes technology that was between 1 and 3 major generations old when it was introduced -- its even further behind now (the next-gen Atoms are more modern, notably they incorporate out-of-order execution for the first time, and will come in 4-core SKUs).
If one isn't moved by the portability argument at all, absolutely build yourself a nice desktop, but if portability is a requirement or simply a nice-to-have then you can have a "good" laptop for the price of a "really good"--but not "enthusiast"--desktop. If you can have both, more power to you.
