I have not heard of one yet but I would be willing to be that when one comes out it would be Seidio along with a bigger battery door. Seidio is BIG on the batteries for BBs.
they have come up with a new technology for lith-ion batteries. instead of liquid electrolites they use some kind of gel. It can hold like 10x more juice and be 100x thinner..... dont quote me on this though, you might want to google it.
But i dont think its gonna be out any time soon. and if it does then well, you know how slow RIM is about stuff.
Other than that i dont think there can be a bigger mah battery without the battery itself getting bigger...