Since the new iPhone will be much faster for which iPhone you will develope? Or you will make two version of your game?
If you mean 3g S is faster then 3g, well, the ipod touch 2g is faster then all models of iphone so having the S model come along doesn't really change anything.
Actually it is different. Namely openGL ES 2.0 support. Problem is (and as a developer for a studio) it's going to be difficult to support a smaller iPhone 3GS installation base. Anyone who has jumped on the 3G bandwagon has to pay through the nose to upgrade or wait until their contract allows it. Honestly I doubt we'll support the new hardware for at least another year + The real game changer is the new 3.0 OS.
The S model is graphically a generation shift from the earlier models. In grossly simplified terms, any iPod touch or iPhone before the 3GS is a GeForce2, where the 3GS is a GeForce6. Note that this comparison is only between their features, not relative performance difference (which is unknown). In other words, older iPhones and iPod touch devices run a fixed-function rendering pipeline, while the new iPhone is capable of running shaders. It will be interesting to see if many developers start taking advantage of the new capabilities, since only a subset of the entire userbase will be running that hardware.
OS 3.0 update will be available for iPones for free, so, may be, there would be more users, then only owners of iPhone 3G S