Hmmm, if you just make sure that the home screen looks remotely like the original ROM, I doubt they'll go any further even if they do power it on. I'd highly doubt they'd think to check software version and even so, they'd have to realize that it shouldn't be 2.1. With all the stuff about the Droid getting it, it would likely slip by.
Of course, if the worst happens and they did notice, you could always play dumb..."WHHHAAATTT??? You're telling me that somehow there was an unofficia,l unreleased, buggy update that downloaded itself on my phone?!?! No WONDER it was acting up! How in the world could that have happened? Now I DEFINITELY want to return it!"
But I highly doubt they will.