Well, as long as the GamerTag that bought the XBLA game is currently on the hard drive, any other GamerTags can play the full version of the game. As soon as you remove the "owner" GamerTag off the hard drive, though, the other GamerTags will only be able to play the demo version of the XBLA game.
It's actually a pretty nice system as I transfer my GamerTag to my buddy's 360 quite a bit and it gives him a chance to play the full versions of the arcade games I purchased. Plus, on my 360, my wife can play the arcade games under her GamerTag as well.
And now you know...