I had no issues with my Droid battery cover for the two weeks I used it, but I have heard some complaints that the cover can fall off. I know another reviewer that had no problem with her review unit, but after purchasing she had issues with the new one. So it probably is dependent on the unit you get.
I did come across a nice little video that describes a potential DIY solution if you don’t mind slightly customizing your Droid. Actually in the world of DIY this is really minor. Of course I am not going to take responsibility for any over-zealous mods on your part. If you want to follow these instructions, take care.
Matt from Motorola (he manages the Motorola Support Forum) added a nice comment below about a non-DIY solution. It’s great to hear a company stand so solidly behind their product. He provides this link to get support if your cover is coming consistently off: https://motorola-global-portal.custhelp.com/app/utils/chat