I'm afraid I don't have time (on a deadline) to test this out thoroughly but here is what I've got: I have the Helpbox module installed. On pages where the helpbox module is loaded the YUI editor fails to show correctly. After some sleuthing I realized that the YUI skin class was not being injected into the body element. When I turned off the Helpbox module, voila! the class appears.

I'm running YUI editor 2.6.0 on a local library. This happens with both the dev and 2.33 versions.