didn't state if he was asking about an Altiris server (part of the architecture) or a managed client. I think it's quite clear from the answers here that
a) You can't install Altiris Notification Server on Windows 2008, at least until 7.1 is released.
b) A managed client running 2008 is supported
c) Running a site server (task or package) service has some support,but not all.
In any case, if we could put a big bold sticky on top of the forums, it would be KEEP checking the
Platform Support Matrix article - Altiris KB46349.