Quite easy really... Though my awnser might not be 100% in line with what they did it will be close enough to make a simuler or same effect.
I cant tell you how they did it 100% but thats only because i dont or have ever worked for ID
Anyway the process for high end game models is qute simple...
1: Model a high poly high detailed charactor or game model. This would hold all the major details for your model.
2: Build a low poly version of this model for use inside the game. You can do this from scratch or by manually optimising your high poly mesh. For high detailed charactors in modern and next gen games thats between 4k and 12k. 12k being the large monsters in next gen like Unreal3. And only when theres not many of them on screen. Usealy 1 or at the most 2.
3: Next you need to setup the UVs of your low poly model.
4: Next you bake the normal and hightmaps onto your low poly UV layout using your 3d programs tools. In Max you use the Render to texture tool.
What this does is take all the shape and hight details off your high poly mesh and projects them onto the low poly mesh.
5: The fun and teadious bit all at the same time. You draw your textures. Taking a copy of your normal map and UVwireframe [witch you can get through texporter] Load the up in photoshop and lay the wireframe over the normal map... At this stage your just using the normal map as a guide to help you see your model in the 2d UV layout. Basicly draw your textures. It always helps to use fabric prefabs for things like fabric and weaves... Or use your
PS tricks to create the same effect. Once you got your defuse/Colour map you then need to save it... And then save it again as a copy. Rename it to "modelname_spec" or somthing _spec just indicating to you its your specularity map. Set the image to greyscale mode.
Let me explain a bit how this works. Like grey scape bump maps the black and white indicate highs and lows. In specularity White indicates high light reflection so it shines more. Black means no light reflection and it would not shine. Verious shades of grey mean the difference between the two...
For plastic youll use a higher shade for fabric a lower shade. I hope you get the idea...
6:You then load all this into your 3d program and apply it to your low poly mesh. You wont be able to see mesh using its normal map/Defusemap/spec map in real time unless your program has one of those lovly full engine opengl preview plugins...
Thats pritty much it... There are some other baking attempts you can do to add extra shadows and things to the defuse but thats up to the artist.
I hope this helps.
Nutsy