I just did something similar last week. I popped a 4 year old LG dumb phone sim into a 5 year old Blackberry Curve 8300. Everything worked perfectly. Two days later, I called AT&T about a billing issue and they noted the LG had become a Blackberry. I'd turned off all data but, while I was on the phone with them, I turned it on to sync the calendar in an effort to remove a gazillion duplicate appts. They told me the system put data on the line right then and there, but I think they did it. The data was on for 2 days and no problem until I called. They said something about when the system sweeps the towers for cell access, it would have discovered the BB. I don't think that's possible, but I can't say why. I just felt that they weren't being honest with me.
I don't know if they'll ever discover it unless the increased data sends up a flag. I use a lot more data with the S2 than I did with my Bold. If you're on a grandfathered unlimited plan, I would notify them because you have nothing to lose. It would help with the warranty, as well. If you're not on unlimited data, I would guess the worst thing they could do would be to change your plan to tiered data.