I just upgraded to 2.3 on two blogs. Immediately, both started showing "Premature end of script headers" on their index pages in the error log. This is a real problem on one blog, because it's also is generating huge "core dump" files I have to delete every few hours to avoid running out of disk space.
I've upgraded all my plugins and removed any that were on the incompatible list. I deactivated a few plugins that supposedly ARE compatible (or don't appear on either list as compatible or incompatible) such as WP-cache, Google Sitemaps 3.0, and Ajax Edit Comments 188.8.131.52. This has reduced the number of errors, but not fixed it.
Any suggestions would be appreciated. My permissions settings (which are the same as they were on 2.2) are 755 for folders and 644 or 755 for files.