I don't know when, but I hope it soon!!!
You should also try Gangstar: Miami Vindication, which is a gta game from gameloft.
Gameloft is able to update the Gangstar series once a year, but I don't know how often gta will be updated. Their one year mark from releasing chinawars was mid January of 2010, which has already passed.
I hope they port the other gta psp game, gta: las Vegas stories, to the iPhone. (not sure if thats the right name for the psp game)