I updated five Community Server 2008.5 sites to SP1 this morning and everything went pretty smoothly. Fortunately I'd done it a bunch of times before. I did encounter a weird "Fix for given key not present in the dictionary error" when I tested the editor's Insert Media function on a custom blog theme.
The popup window displayed the Oops-masked "Fix for given key not present" exception and not the insert media form. I've come to like using that little widget, so I had to investigate.
Seems there was a new area added to the blog theme.config in the SP1 pertaining to media that wasn't in CS2008.5, so I obviously missed a file compare.
So kids, don't make the same mistake I made. Check those custom blog theme.configs when you upgrade to CS2008.5 SP1.