WordPress 2.5.1 follows 28 days after the initial 2.5 release – it is an essential update that contains a security fix.
To quote Ryan over at the Official WordPress Blog:
It includes a number of bug fixes, performance enhancements, and one very important security fix. We recommend everyone update immediately, particularly if your blog has open registration. The vulnerability is not public but it will be shortly.
You are able to just replace wp-includes/pluggable.php, wp-admin/includes/media.php, and wp-admin/media.php to fix the security issue, although 2.5.1 also includes a number of other bug fixes, so a full upgrade is recommended.