Isn't "antigravity" a mod for UT2004?
It was, however after we realized that there just wasn't a way to make wallwalking work properly and still have an enjoyable game, We decided on an engine change.
Wallwalking in the q3 engine can suck too, because bounding boxes cannot be rotated. Unless you do something very clever indeed, only cubic player entities will be able to wallwalk.
edit: it should be mentioned that what I say applies to Tremulous, and I'm not entirely sure q3 is restricted in the same way. Probably is, though.