Got the phone yesterday love it!
I pulled the battery and looked at it.
Incredible: Model BTR6300B 3.7VDC 1300mAh 4.8l Whr
Eris: Model BTR6200B 3.7VDC 1300mAh 4.8l Whr
They are essentially the same battery. The connector +/- tabs are in the same place and the dimensions are the same.
So I took it step further and tried my Seidio Innocell 1750mAh battery I had for my Eris. It has been in my Incredible for 24 hours and working great.
Oh and a side note the back snaps on!! No space like it had with the Eris. Though I did file the edges down alittle when I was using it for the Eris.
I will post up some pictures if ppl need them. And if nothing else those with Eris's just laying around now have a spare battery for their Incredible.