Actually, I believe that they do have the same battery chemistry. The iPod Touch and Iphone 3g uses lithium-ion while the iphone 3GS uses lithium ion polymer chemistry. Since the iPod touch 2G came out last year, it is quite possible that the iPhone 3G and the Touch share the same chemistry.
Both are quite different in their voltage and operating characteristics despite having lithium in it. Lithium ion polymer allows for longer operational life and larger charging cycles while maintaining its charge-discharging cycle.
You can see more info on Apple's website
here.
I wonder if iPod Touch 2G and iPhone 3G share the same battery.
Also, how long does your 2G iPod Touch's battery last?
