We have just finished successfully testing the Joomla 1.5.15 package on the 2009.10 release, and have updated our package archive to include it.

Joomla 1.5.15 is quiet a large update, and includes moderate security fixes. Due to the large update, and the fact that the security fixes are not considered critical, appliances will not upgrade automatically for stability reasons. You should upgrade at your earliest convenience:
apt-get update
apt-get install joomla15