The battery is 60 watt-hours, the same capacity as the 13" MacBook Pro.
The previous plastic MacBooks featured a 55 watt-hour battery and claimed a 5-hour run time. Apple has added 5 watt-hours of battery capacity and two hours of run time. Either this machine is substantially more efficient than its predecessor (thanks to the LED backlight?), or Apple's new battery life claims are overly optimistic.
Despite its higher capacity, this new battery is actually lighter than the previous MacBook's battery. This battery boasts a power to weight ratio that's 23.5% better than its predecessor.