I have a Pearl (8100) and a curve (8310). The standard 8310 battery (1100 mAh I believe) and the Seidio 1500 mAh extended battery for the Curve both fit in the Pearl just fine. The pearl battery door has a slight bulge when I do so but its not a big deal. The extra 200 juice is worth it.
Th 8100 battery (900 mAh I believe) also works on the 8310 without any problem.
Another note - the stock battery on the 8310 is the same one found in the 8700. The 8700 did not like the Seidio extended 2600 mAh battery for the 8310 but was happy with the Seidio 1500 mAh one.
9700 on 6.0.0.something, BES 4.1 on Exchange, BIS (AT&T)
Ex 8700, 8800, 8100, 8310, 9000