Actually, the number hasn't ported in--I haven't initiated it yet, for a variety of reasons.

Tim mentioned the last 4 of SSN, probably because carriers have been slow to allow the user to customize their account password. Verizon, for example, allows (and I think encourages) customers to set a unique numeric pin to get away from the SSN verification. As for prepay, those numbers are portable, you just need to get the "account number" and "passcode". Some carriers use the phone# as the acct#, others require you to call CS and ask for it. (I'm not sure how the prepay carriers verify your identity.) As for passcode, some carriers use the last 4 digits of the first PIN you activate on the phone, for example. Others use the last 4 digits of the phone number (not exactly secure, but the passcode can generally be changed by calling the carrier's CS).