Virgin Mobile is a prepaid division of Sprint that offers unlimited data to all of its users. On March 23rd the unlimited data will be throttled when reaching certain markers.
While Virgin Mobile will still offer its members unlimited data they will only get the 3G speeds until they hit 2.5 GB of data use and the speed will drop to a creeping 256kbps after that until the end of the month.
Sprint being the Corporate part of Virgin Mobile using most of their marketing to talk smack about other carriers that throttle their speeds (ironic right?). At least for now Sprint is not saying anything about throttling their unlimited data plans and since this is their claim to fame for the iPhone 4 we assume it wont happen any time soon.