WordPress 2.3.3 has been released as an urgent security release.WordPress blog says that this release is mainly to fix vulnerabilities in XML-RPC.
This vulnerability could let exploiters to maliciously alter posts in your blog though well constructed requests.Wordpress 2.3.3 also fixes a few other bugs.
The files that have undergone changes in wordpress 2.3.3(from the earlier version – wordpress 2.3.2) are
If you want to fix only the vulnerability in xmlrpc then you can download the fixed version of xmlrpc.php and copy it over your existing xmlrpc.php.
Else you can download wordpress 2.3.3 and replace only the aforementioned files.
Not sure how many more urgent security releases that wordpress will have to make in future.