The IRS doesn't send 1099's. If they sent you a notice it was because ING sent them the information on a 1099. Maybe it was online but they did not mail it to you?
I've had plenty of sign up bonuses that I never gotten a 1099-INT for. The only reason I mention it, is because INGDirect never sent me one, but the IRS did, and generated a CP2000 IRS notice on me.