It's actually pretty good on Apple's part -- release another product, make a nice amount of money, wait a few months and release the same thing with a camera then they get more money. Then next September, throw in GPS and everyone with a 2G, 3G, 3G w/ camera iPod Touch will upgrade to it.
Apple knows how to milk their customers out of some money. No matter what you say or think about Microsoft or any other software company, they all pale in comparison to Apple ... they COULD have released the best iPod with all the features of the iPhone without the phone, but they're not going to do that because it wouldn't rake in nearly as much cash if they progressively added features from the iPhone. It's perfect business sense, but not really customer friendly (IMHO).
I don't think we'll see many people moving from their 2G iPod Touch to a 3G (or even to a 3G w/ camera). However, if they throw in GPS that would probably get both 2G and 3G to upgrade if they make the case well enough ... which Apple does pretty well.