I spent the best part of this morning upgrading Wordpress on this and my personal site. My personal site upgraded without any problems (that I've noticed), however I encountered problems here.
Firstly, I lost all of my pages. No idea where they went, but they just disappeared. Luckily, I only have a few so they were easy to re-create.
The next problem I encountered is with my theme. It appears that if you call query_posts() within "the loop" (for multiple loops), then is_home() no longer returns TRUE when you're on the home page. This was quickly resolved by implementing one of the other methods for using multiple loops.
Whilst this is fine for me, I think this is a bit of a silly change for non-techie people. Suddenly their themes are not going to work as they did before, and they're not going to know why. It's also an undocumented change in behaviour.
If you spot any problems, please let me know in the comments.