Read through this thread:
http://forum.ppcgeeks.com/showthread.php?t=98048
In short, if you were formerly an Alltel customer who is now a part of Verizon due the the merger, and you're still using your old Alltel plan, you can purchase a Droid on eBay or for full reatail and activate it on that plan. That's what I did (the eBay route). If you go into a store or talk to a Verizon rep on the phone, they might tell you that you can't do it. But you can. The easiest thing to do is just to activate it online through your My Verizon account. No problems whatsoever.
But, if you're still an Alltel customer now who accesses your online account through Alltel, and you're not a Verizon customer, you can't do it by normal, legal means. Anyone who is still a part of the Alltel network (receiving Alltel bills and logging in through the Alltel websites) at this point was not included in the merger (or hasn't been incorporated yet), so it's a completely different company. It would be essentially like trying to activate a Sprint phone on the Alltel network.