Virgin: No. The GS2 hardware is incompatible with their network. I believe they use Sprint's CDMA/WiMAX.
T-Mobile: Yes, you'll be able to use their pre-paid service. But, which GS2 do you have? Original i777 or i727/Skyrocket? Reason I ask is that if you have the i777, you will only be able to get 2g/EDGE data speeds. However, the Skyrocket can use T-Mobile's 3G network if you do a little "hacking."