Strange data plan speed on Nexus S I have a Nexus S and a GPRS plan activated, from what customer services of my mobile carrier have told me is that GPRS will give you max of 10-15 kB/s download speed.

In my Nexus S if i don't select "Use Only 2G" in "mobile network" setting i get speeds up to 80kB/S and the symbol in the status bar shows 3G symbol. As soon as i select the "Use only 2G Network" my download speed drops to 10 kB/s. 

I have used spedtest application to check speed.
I have verified with my Mobile carrier and i do not have any 3G plan activated.

How is this Possible, i don't want to get any kind of shock when i receive my Mobile bill :)

Edit : My mistake, corrected capitalizations.
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.