If you don’t keep tabs on your blog’s dashboard very often, you may not have noticed that a new minor version of WordPress was released earlier today — WordPress 2.6.5 — which I encourage you to take a few minutes to apply to your site(s) today.
The major changes which have been announced are below, but you can view a complete change history at WordPress Trac.
Protection against a potential XSS exploit.
delete_post_metahave been updated to ensure proper functionality when working with post revisions; ensures that post meta, such as custom fields, are attached to a post itself rather than to a revision.
Updates to XML-RPC to ensure working with only the correct post types.
Fix an XSS issue with Atom and RSS feeds on certain hosting configurations.
(Source: WordPress 2.6.5 in Detail)
Upgrading is quick and painless, especially if you use the Automatic Upgrade plugin, so get it done as soon as possible!