I finally updated from 2.1.3 to 2.2. Afterwards, I'm finding that the theme keeps reverting back to the Default theme and undetermined intervals.
I had a bunch of plugins, but whittled it down to the following:
Akismet 2.0.2, Bad Behavior 2.0.10, Disable WordPress Widgets 1.0, FeedBurner FeedSmith 2.2, Full Text Feed 1.03, Remove Dashboard Feeds 1.0, Subscribe To Comments 2.1, Taragana’s Del.icio.us mp3 Player Plugin 1.0, WordPress.com Stats 1.0, WordPress Database Backup 2.1.2
I tried K2 v0.9.6 and Barthelme 3.0.1. It still happens.
I saw bug ticket #3907 "WP reverts to default theme on file access collision".
(My blog post about all this here.)
I clearly see other WP 2.2-based blogs that are just fine. What else can I do?