As was announced over in the CS.org forums yesterday, Telligent has released Service Pack 1 for CS 2.1.  A list of fixes/enhancements as well as download and installation instructions is located here (you can choose to only download the SP itself, or an complete installation of CS2.1 with SP1 included).  I have updated most of my sites (including this one) with no issues.  Of course if you've modified any of the core CS code you'll need to run a diff on those files and download the SP1 SDK so you can modify the appropriate code.