Today I found some major problems with PHP >=5.3 and a few minor problems with Joomla 3.1.
The major problems:
In PHP 5.3/5.4 there are some major changes that make uddeIM incompatible with these PHP versions. Depending on which uddeIM functionality you have enabled, uddeIM might run without problems on your system or not. It will not crash your site but using one of the broken functionality is simply not possible.
The minor problems:
Joomla HQ breaks with each Joomla version the backward compatibility of the Joomla framework. In Joomla 3.x there are so many changes that I missed a few. For example uddeIM 3.0 cannot get the site name of your Joomla site and so it is missing in notification messages.
You can pull the current development version from the SVN on Joomlacode.com (see other sticky). This will fix most (hopefully all) problems.
uddeIM & uddePF Development
CB Language Workgroup
CB 3rd Party Developer