You may not like the answer but I would say XML is your best bet. Conditions can simply be child nodes to the potential dialog versus, your dialog engine can then just compare the conditions to select the best line and go from there. Might be a bit easier to show with a bit of XML, the below is pretty vague and ambiguous but might give you an idea on how to adapt and use a technique like this. If you would like some more design ideas get in contact with me directly I might be able to help you out.

<root>
    <dialog id="1">
        <condition name="whatever" value="mustmeetthis">
            <say>Hello!</say>
            <answer id="1">
                <respond>Wussup?</respond>
                <condition>
                    <goto>dialog ID here</goto>
                </condition>
            </answer>
            <answer>
                <respond>Why are you talking to me?</respond>
            </answer>
        </condition>
        <condition ...>
            ...
        </condition>
    </dialog>
    ...
</root>

XML shouldnt be messy, it should be hidden by forms and such that you have to add stuff... so you are kind of contradicting yourself when you say

"What I want is some method that allows me to create a dialog editor" and then "I tried an xml based approach, but it was too complex"

there is a post earlier about this topic with some imagery, IE RPG maker, check out how they represent conditions.

I know nothing about ruby, so I can't help you there.

Well, I mean by complex that the XML layout was messy and hard to parse, perhaps because it was incorrect from the beginning. Im back to the drawing board using Dan's sample, maybe this time I can get a working result.

There are an abundance of premade libraries out there that can parse the objects for you. Simply look for the language you want and "XML Parser". If you want to build your own it's actually pretty simple if you take an anonymous object oriented approach. That is to say make a base XMLNode object that can have a collection of properties, child nodes and an innerText or data property. Then you simply read straight through the file and set the property NVP's and data to the XML Node object. Then step into each of the children and repeat, just make sure that you can bounce back to the parent object when you are at the end of a child object. For a bit of a sudo code demonstration the above example I gave should populate into an object hierarchy something like...

XMLNode (root)
  .props (empty)
  .children
    [0] XMLNode (dialog)
      .props
        [id]='1'
      .children
        [0] XMLNode (say)
          .innerText = 'Hello'

        [1] XMLNode (answer)
          .props
            [id]='1'
          .children
            [0] XMLNode (respond)
...

XML Itself is pretty standardized, you just have <NodeName property='value' moreProps='moreVals'>innerText <childNode></childnode></NodeName> and so on, using prebuilt libraries you can easily query and map values along these lines. You could even extend or inherit from the XMLNode into a more specific node that might contain the values under a more precise naming convention that makes more sense to your code. Not to be that guy that tells you to go read the manual but you might want to do a bit more research on what XML actually is before considering it overly complicated.

I'm unfamiliar with Fallout's dialogue system, but I thought I'd share what I've done and maybe it will help you get some ideas. Our dialogues are all constructed in Lua scripts. I wrote a handful of C++ classes to handle the data, display, and execution of dialogues that is re-used across different areas of the code where dialogues take place. These classes are sometimes extended (via inheritence) to more advanced classes when a piece of the game needs to add additional features and data onto a dialogue. The C++ code is bound to Lua via Luabind at run-time.

Here are some files to take a look at. I won't go into detail about how everything works unless asked. But the code is farily well documented and easy to understand, so hopefully that's enough to get you by.

- Header file for dialogue classes (best source for documentation on how this all works)

http://sourceforge.net/p/allacrost/code/2064/tree/trunk/game/src/common/dialogue.h

- Implementation of the dialogue classes

http://sourceforge.net/p/allacrost/code/2064/tree/trunk/game/src/common/dialogue.cpp

- Example of a dialogue being constructed for use in a battle:

http://sourceforge.net/p/allacrost/code/2064/tree/trunk/game/dat/battles/first_battle.lua

- Example of a dialogue on a map. This file has a lot going on, so just search for "Dialogue" and you should find the relevant parts to how that is done.

http://sourceforge.net/p/allacrost/code/2064/tree/trunk/game/dat/maps/opening_scene.lua

Hope that helps you get some ideas. Let me know if you'd like any clarification on any of the above. Good luck!