Activate Used iPhone 4 If I were to buy a used iPhone 4 (e.g. from eBay), which carrier(s) would allow me to activate it without a contract?  Is there an activation fee?  Anything else I should be aware of?

UPDATE:
It seems there is less knowledge as to what Verizon's policy is, and as luck would have it they're my current provider.  I would like to know what their policies are also, if anyone happens to know.
So long as it's an unlocked iPhone, you can use any SIM card to activate the unit. It doesn't actually have to have a plan attached to it. I'm not aware of any carrier on the world that only supplies contracts and foregoes monthly plans, so there should be no issue with your carrier of choice. Contacting Verizon is a wise idea with respect to their policies, as without further details your question is pretty broad.