The battery. Put your soldering irons away, they won't be needed! Apple part #616-0372.
The recycle marker on the battery is blacked out with a sharpie. Suspicious...
We were all expecting a bigger battery, and I can't verify this, but this page references the part number on the battery and lists a capacity of 1150 mAh, not the 1400 mAh in the original. Can anyone dig up more information?