The planned inter-operability of future versions of Unreal with Epic's upcoming sequel, Unreal Tournament, has been mentioned often, since the two programs will share a similar code-base. PlanetUnreal has reproduced a Mark Rein newsgroup post that clarifies how interchangeable maps from the two games will be:

UT will run most Unreal DM maps but probably not run single player maps.

For multiplayer UT can serve most Unreal DM maps (provided you have the texture sets etc.) with any problems but only UT users can play on a UT server even if it's playing an Unreal map. I.e. you can't connect to a UT server with Unreal or an Unreal server with UT.