WordPress 3.1.1 is now available. This maintenance and security release fixes almost thirty issues in 3.1, including:
Some security hardening to media uploads
Performance improvements
Fixes for IIS6 support
Fixes for taxonomy and PATHINFO (/index.php/) permalinks
Fixes for various query and taxonomy edge cases that caused some plugin compatibility issues
Read more here ht*p://wordpress.org/news/2011/04/wordpress-3-1-1/