It's certainly possible, with at the very least 1X speeds. Both companies are CDMA and use the 800/1900MHz frequencies, so you may even be able to use 3G if you were to unlock the Droid and use it on Cellular South.
I had my number ported from cs to Verizon no problem. I doubt if they will let you run a Droid on their network. But it want hurt to ask. Your going to love this phone.