When I bought Parallels (4 copies actually), the system requirements stated 10.4.xx or later. I find it really upsetting if I am forced to pay more money for the same software to keep it working when the operating system receives updates. Especially when there are should be no no issues in keeping it from working (3.0 claims to already have the internals in shape for supporting 10.5 - they are probably fairly easily backported to 2.5).
chabig,
We will wait for the final Leopard release which will be available on Oct 26 both for us and our customers. We expect that 2.5 will work on it in general but will probably have a number of issues. Then we will fix issues and release 2.5 update in few weeks depend of the issues found.