Here's a great factoid from iFixit's teardown: the iPad power source is made up of two batteries that are each roughly the same size as that found in the iPhone. The kicker? Each has 2.5x the capacity. UPDATE
Overall, the iPad has 5x the power capacity of the iPhone.
Why is this interesting, beyond being the king of your resident water cooler? It's not hard to imagine the next iPhone being equipped with one of these iPad batteries, meaning that even if you don't give a crap about the iPad, your iPhone 4G could have 2.5x the run time.
Of course, if you don't care about the iPhone or the iPad, well, why are you reading this? [iFixit]
UPDATE: We misread iFixit's sarcasm. The iPad batteries are indeed quite a bit bigger than iPhone batteries. From iFixit:
4.5 mm x 118 mm x 57 mm for each of the two cells
60.5 cc volume total
4 mm x 41 mm x 68 mm