You can play all iPhone/iPod Touch games on the iPad. The thing is some developers make their games universal meaning it is designed for the iPod and iPhone but it is also enhanced to look better on the larger iPad screen and it is all included in the one download.
Other developers release separate apps such as an HD version so while you can still play the original game on the iPad if you bought the HD version it would look better because they optimized it to run on the iPad.
I'm assuming Rockstar is going to release an HD version of Chinatown Wars and charge more than the $9.99 price for the origial version meaning if customers own the original but want to play the HD version on their iPad they will need to buy the game again. They may also just update the app to make it universal but for some reason I doubt that will happen.