As you've probably seen in the blog WordPress 3.1 Beta is out now.
Please be sure to reference this known issue if you attempt use of it.
I'll start with a known issue that many of you are likely going to see:
Fatal error: Call to a member function is_page() on a non-object in
is_page() may be any conditional tag from
Here's the issue: A theme or plugin is doing something wrong. Some code is checking the value of a conditional tag before we actually set up the Query, which means they don't work yet.
If you like, you can report issues you find in a new thread.