WordPress team just release the version 3.1 on 23rd February which has lots of new features like admin bar, redesigned linking workflow and much more. WordPress 3.1.1 is a maintenance and security release of version 3.1.
As mentioned in official post:
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