Perhaps Never.

In an interview with Nintendo Power Magazine Dan Houser has stated that GTA on Wii "didn't feel natural" and that is the reason the DS was chosen.

"It didn't feel natural to us, I guess. It really was that the DS felt like it had a lot of interesting challenges that would be totally different from what we'd done in the past," he told Nintendo Power when asked why the series wasn't moved to Wii. 

"The stylus and the chance to use mini games in that way was really interesting and exciting to us, and we thought we could integrate seamlessly between those two modes. And it would be the chance to make something really good on a handheld with our handheld-focused team.

"That was really why we went that way. We haven't really done any concrete, major thinking about the Wii, one way or another. They're sort of separate issues," said House

Another major flaw of the Wii when it comes to GTA would probably be the control. I'm sure anyone who played Driver Parallel Lines would agree that the game just didn't feel right. 

Source