I just asked a Verizon rep this same question at a local store and he told me no. Verizon does not allow outside phones to join their network.
This is unfortunate, however, there may be a way for an outside phone to joing the network if it appears to be a Verizon phone. There are several packages available online that will reprogram phones.
They announced that later in 08 this will be possible. They still want to make sure it's going to work. I would expect any device that runs on CDMA should work, so PDAs should be OK.
as far as i know alltel phones are capable of working on verizon. i know my last phone was alltel and it worked. also verizon is buying out alltel by the end of the year.
As long as it is a CDMA phone it can be activated on Verizons network a sales rep might give you a hassle but they will do it for you. Awhile back I activated a Nokia that wasnt available in the states with hassle but it worked fine in the end