In early June, Joomla 1.5.11 was released and I've been testing it for a few weeks. I highly recommend upgrading your Joomla 1.5.x to the latest version. Here's the announcement from the Joomla Project
The Joomla Project announces the immediate availability of Joomla 1.5.11 [Vea]. This is a security release and users are strongly encouraged to upgrade immediately.
This release contains 26 bug fixes, two moderate-level security fixes and one low-level security fix. It has been 11 weeks since Joomla 1.5.10 was released on March 28, 2009. The Development Working Group's goal is to continue to provide regular, frequent updates to the Joomla community.
Download your updates here
Click here to download Joomla 1.5.11 (Full package) »
Click here to find an update package. »
If you don't want to install your own updates, we can do it for you for only $50 until the end of June. We normally charge $125 for this service. You must be running a version of 1.5.x not the original version of Joomla 1.0.x.
1. Back up your current Joomla 1.5.x website including the database
2. Install the Joomla 1.5.11 version
3. Test your website to make sure everything is working properly.
Click here if you want us to install your Joomla 1.5.11 upgrade.