Till a couple of days now I was using Joomla! 1.5.5 but yesterday a new release came out.
This new release is a security release made to solve a high level security issue, then it’s recommended to upgrade immediately.
Some websites are already being owned because of this security flaw, do not risk to get owned too and update your website software as soon as possible.
If you need help read the migration/upgrade instructions in here.