Before you buy a replacement battery for your dead Roomba, you might want to consider rebuilding the pack yourself. You can get a couple of 6 cell Sub-C NIMH packs for less money and more amp hours than the stock battery.
I’ve been wondering whether it would be economical to just rebuild the battery pack, versus buying a new one. From the looks of it, all it takes is 12 NiMH cells, and from the looks of the case, it looks to be C-Size form factor. For less than $36, I can pick up 12 4500 mAh C cells. I can see iRobot has tried to prevent people from doing this by using TRIANGLE bits for their battery cases, but that’s simple to get through.
To remove the triangle bits, just find a small flathead screwdriver that you can wedge in nicely.