Joomla 1.5.23 was recently released (Apr 5, 2011) and we recommend that everyone running Joomla 1.5 upgrade their version to Joomla 1.5.23 A.S.A.P. Joomla 1.5.23 is a security release and contains a number of bug fixes as well.
Click here to see our tutorial on how to properly upgrade your Joomla site to Joomla 1.5.23.