As usual, the fine folks at iFixIt immediately took apart the newest piece of Apple hardware to arrive at their offices, a unibody 17" MacBook Pro. The whole process was detailed with step-by-step photos, some of which reveal an internal battery roughly the size of a baby dolphin.
In case you're curious, and we know you are, that's a 7.3V 95Wh (12820 mAh) battery, Apple model #A1309. It contains no mercury (Hg) and weighs in at 1.5 pounds (0.7 kg)! The folks at iFixIt note, "It's only a matter of time until you'll be able to buy this battery online from companies like us."
It reminds me of the battery that came in the clamshell iBooks (in size at least). It sure looks easy to replace...