Can my iPhone work without a SIM Card?

My friend gave me his old iPhone because he is going to be getting the new 3G one when it comes out. I'm not with AT&T and never have been (or with T-Mobile who also has SIM Cards), and so i was wondering if I can use the iPhone without going into AT&T and signing up with anything or getting a SIM Card. Is there any possible way I could activate it with the current carrier I have which is Cricket or use it without activating it or getting a SIM Card. And if it is possible, what would the phone number look like when i call someone or if someone wants to call me?