id's John Carmack updated his .plan with his response to a question posted to a Mac OpenGL programmer's mailing list dealing with "display lists" and how OpenGL handles them. It's very technical in nature, but here's an excerpt that explains a bit of what he's talking about:

In a busy Q3 battle, the triangle count may be split roughly evenly between character models and world geometry. Going from an empty scene to a pitched battle can result in a 50% performance drop if you are triangle limited. Not great, but livable.

If we kept the same ratios and designed for geometry acceleration with all the static world geometry in display lists, then the empty scene could have 4x the geometry and still be running the same speed. However, current OpenGL display lists can't really accelerate high quality skinned characters, so when an equal number of character polygons was in scene and passed through normal direct rendering, the performance would drop to 20% of the original. Unacceptable.

So, either you would have to use significantly different polygon counts in characters and the world, or some new API features would need to be defined. Nvidia has a skinning extension that gives some benefit, but still requires a character to be broken up into one static list per bone pair, instead of a single list for the entire character.