Nearly a month ago WordPress released 2.1 Ella, which considered to be the biggest version update of WP history, since 1.5.2. However WP team continues its efforts to bring up bug fix and additional features. In that on going effort, yesterday WordPress release bug fix and security release for both of their actively maintained branches of WordPress.
Version 2.1.1 includes about 30 bug fixes, mostly minor things around encoding, XML-RPC, the object cache, and HTML code. on the other hand Version 2.0.9 only includes the security update, which was around the code to prevent XSS. Please take not that WordPress team will release security updates to 2.0 branches as often as it’s require till 2010, but all the new features and development are going into the newer branch(currently WordPress 2.1).
This is a low-to-medium priority update recommended for all WordPress users.
For complete list of changed file and download please consult with WordPress official release post.