Certain stores will, most won't. I was living in the Detroit Metro area at the time and I had picked up the Droid from Craigslist. The ESN itself was good for Verizon, so I took it to the local MetroPCS dealer.
They didn't want to at first, but I purchased the Ascend for my girlfriend. After that, he was like... Sure.
I have since sold the phone. I moved up towards Flint and MetroPCS's service up here doesn't yet support the Droid. In fact, a lot of users are screwed up here. Metro's service is so bad, user's can't have the Esteem. Stores don't even sell them. We're stuck with the Optimus, or the brand new HTC Wildfire S (Basically, the Optimus with a better camera.) for smartphones.