Strange data plan speed on Nexus S
OK, so it's quite obvious that your phone is receiving 3G service (likely EDGE/UMTS), not just the 2G GPRS service.  GPRS is simply not capable of the speeds you're getting.  Since you've called the carrier and had them check, it's their problem.  I would say just go with it and enjoy the free service.  Don't let them charge you for it if they figure it out later, since you did try to make them aware.  If you're worried about it just call again and make the situation really clear.