If they ported San Andreas to the wii and it was $30 I would probably buy it.
That said seeing as they are spending all this dev time on the GTA4 engine they will milk the engine for a couple more games. Only after they've released a couple sequels to GTA4 will they consider a wii version I would say. I'm not sure how manhunt and bully sales went. But I highly doubt they will be good enough to convince rockstar to make a wii version.