I have the same problem, I have an iPod Touch right now & really want Sketches, however I don't want to pay for it again when I get my iPhone 3G.
So by the looks of these posts you can put apps on more than 1 iPod Touch/iPhone right? If so, that's just like music, as you can transfer it to as many iPods so long as they always sync with your computer only.
I hope this is the case, because I am just about to purchase Super Monkey Ball right now. Please tell me if this is not the case.
SuperMacMan