Just set up a test blog on my laptop, and everything has gone smoothly execpt for two things:
1. All installed themes are incapable of being activated. WordPress responds with "The active theme is broken. Reverting to the default theme."
These themes work fine on my online blog.
2. The themes are not editable in the Theme Editor, at the bottom of the page it says "If this file was writable you could edit it."
Obviously i have something setup incorrectly on my server. Whether is a permissions thing or .htaccess issue or whatever, i'm not sure. Perhaps these two issues are related.
Running the 12/11 nightly, also tried 12/10 nightly.