I have the Seidio 2700 mah extended battery on my Droid1. It works fine - the battery, is what I'm talking about - however be advised that Motorola's hardware appears to have a problem getting accurate information on OEM batteries, and it's difficult to get truly correct readings of the battery's available charge. Somewhere, here on Droid Forums, is a massive thread devoted to the Seidio battery, and the hoops the user has to jump through to enjoy the benefit of an extended battery. It's worth reading to gain an insight on what you'll face with a Motorola Droid1. This is NOT the case on all android phones, BTW. I have a Seidio 3500 mah extended battery on my HTC Incredible, and it has worked flawlessly since the day I installed it, and the Dinc. has no problem reading the correct charge level. HTC vs Motorola hardware? I don't know, but it's very curious.
-Mike