Some mutators are compatable.
I'm currently sorting through my UT2003 files to see which work and which don't. From my experience, mutators (well, most of) work, skins/models work (but not with vehicle's, your character carries on running instead of sitting etc).
Mods however, are a trial and error situation. Out of all the mods I have tested, so far xCTF is the only one so far.
Deathball has been recoded (still in beta stage though) for UT2004 (
http://www.deathball.net). There aren't many servers with it on, but given the popularity of the mod, loads more are likely to come.
UnrealDemolition (
http://www.unrealdemolition.com/) is one which I'm personally looking forward to. They released a video for it, and it looks really good. On their site there is a download for it, but I can't get it (motherboard screwed up at the worst possible time, although it is giving a chance to continue learning C++).