I just googled for XBox Live features, and demo downloads seemed the most likely possibility for a single player game (last I heard Larian was still undecided about a demo, but that was awhile ago). If you don't need a Live certification to offer a demo through the service, though, I guess downloadable content is a possibility.
I think DLC maybe coming.
My reasons for this if you look at Fallout 3 they have the Xbox Live logo on the box art, even though its a Single Player game yet they knew they were bringing out DLC.
They even put content downloads on the back
My thoughts are there is some sort of DLC planned in the future.
PS.Larian whens the DEMO?