From the Official WordPress Blog:
2.3.1 fixes over twenty bugs. Some of the notable fixes are:
* Tagging support for Windows Live Writer
* A login bug that affected those with a Blog Address different than
their WordPress Address is fixed
* Faster taxonomy database queries, especially tag intersection queries
* Link importer fixes
If you’re new to the WordPress system, I suggest avoiding any release candidates; however, if you’re well versed in understanding bugs and know a bit of tracking errors, click here to download WordPress 2.3.1 Release Candidate 1.