A surprising piece of information came out of a TouchArcade interview with Handheld Games Corp’s CEO Thomas Fessle. Fessle noted that performance of their 3D TouchSports Tennis game (iTunes, US$5) is noticeably different across models and that it runs best on the new iPod touch (second generation):
The most clear difference is that fact that the iPod Touch’s processor was quietly boosted to 532MHz (up from 412MHz) with the 2nd generation model introduced in September. Meanwhile, the iPhone 3G, Original iPhone and 1st Generation iPod Touch continue to run at the original 412MHz.
While you might think that the iPhone 3G would have the faster 532MHz processor, it's not the case. The game still runs on all of the iPhone and iPod touch models but it definitely loads a lot faster on the new iPod touch.
Update: Fessler also speculates that the GPU has also been improved in the iPod touch 2G but he can't prove it. TUAW adds that a better GPU could explain why certain apps only run on second-generation iPod touch devices.