Currently I use a Virgin mobile but I was thinking of switching to a pre-paid SIM (ATT) as the screen is cracked on the phone and auto-top up is not working so I am looking around. A couple of questions:

1) How easy/hard would it be to port my current number to a new SIM, and how long does it normally take?
2) If using the SIM in an unlocked quad-band phone would the local US number work worldwide? Or is there a block on roaming?