Dear Aunt TUAW,
Will my AT&T iPhone 4 work on Virgin's no contract plans announced today?
Your loving nephew,
Unfortunately, it would appear not. Virgin is a mobile virtual network operator (MVNO) for Sprint. Virgin uses Sprint's network infrastructure to provide service, which means you'd need a compliant CDMA iPhone instead of your AT&T GSM iPhone.
CDMA and GSM are two distinct technologies for providing 3G cellular service, with CDMA being popular primarily in North America.
If you want to go with Virgin Mobile, you'll have to purchase a new handset. Today's announcement suggests that prices for those will start at US$549.
Thanks everyone who helped out Auntie on this question.