I believe the DS game was a test in the first place, a prototype for a Wii GTA. They put a lot of effort into it though, because there's no point in testing the water with low quality games.
The PSP and iphone versions are, I believe, further tests (as well as leveraging the work they've already done for more profit). If it flops on those platforms, it was more likely the game and not just a difference of audience.
A game I'm developing with some friends:
www.xnagg.com/zombieasteroids/publish.htm
It is largely a technical exercise but feedback is appreciated.