The one I know about (and I am sure there are more) is that even though you can put iOS 4.2 on a 2nd generation iPod Touch, it will not do multi-tasking. This requires a 3rd or 4th generation iPod Touch.
That I don't know, but here is the way to tell straight from the Apple's mouth --I think, even the 3G 8GB won't be able to do multitasking as it is actually the 2nd Gen.
Right?
The iPod touch (3rd generation) can be distinguished from iPod touch (2nd generation) by looking at the back of the device. In the text below the engraving, look for the model number. iPod touch (2nd generation) is model A1288, and iPod touch (3rd generation) is model A1318.