I have a 3200 mAh battery somewhere. But I really don't like the bulky back. When using Froyo, I had to use it as my phone wouldn't survive 12 hours with the stock battery. Now, running Gingerbread, a 1500 mAh battery I bought from DealExtreme for 5 USD lasts about 20 hours normale use.
You do have to cut away a little bit of the plastic though. But they are cheap, I've got two of them now, one in the phone and one charged in my pocket.
I think this is as big as it's going to get without having to replace the backdoor?
HTC Hero - Cronos Ginger 2.3.3 X
HTC Sensation - TripNiCE IceCream Sandwich 4.0.1 - Alpha 7.5