Everyone talks about userbase alone, but that is not so simple, how much of Wii's userbase is the kind of people that would play a GTA game???
For example, look at Call of Duty 4, it's on PS3, 360 and DS, the 360 version is near 7 million, the PS3 version is near 4 million, and the DS version? less than 200k... and the DS has a near 100 million install base... not always the largest userbase is the people interested in your game...
I think they should do a study to see if it's good making a Wii GTA game... I'm sure it won't be enough to make it exclusive, but probably enough to make it multiplatform (PS3, Wii, 360) and make some more cash...