Serendipity 1.1.3 and 1.2-beta2 have been released due to a SQL injection attack reported by Dr. Neal Krawetz today. It is possible to abuse a 'commentMode' variable to inject SQL code that was targeted to the function that fetches comment information. This variable was introduced to Serendipity 1.1 - all prior versions are not affected.
They also suggest checking you access logs for a "commentMode" variable issued in requests to see if there were any kind of attacks made already. The fix is a simple matter of editing the functions_comments.inc.php file and replacing the line of code they give with the more secure versions. Again, this is recommended as an immediate upgrade for Serendipity users.