I mentioned this somewhere before, but just to note again...
The Al15" does have a new 50 watt-hour battery (viz the new 50 watt-hour battery for the 12" and 58 watt-hour battery for the 17"), but you can only buy the old extra rechargeable batteries, rated at 46 watt-hours (which is even less than the old 47 watt-hour batteries that you could get for the rev B 12", which don't work in the new rev C 12", and are now going for half the price).
Unfortunate and a little disappointing. But anyway... ultimately the extra battery decision comes down to how often you find yourself on the road without juice in your Powerbook or access to a power point.