Using $10 unlimited data for HTC inspire?

I have read blogs about people getting the $10 ATT non-smartphone data plan and buying Androids off Ebay and such and using the internet on their new Android phone with the $10 unlimited data plan because ATT doesn't notice that they changed phone because they did not purchase the phone through ATT. Is it true that you can do this? I was about to buy an HTC inspire off of Ebay and use my Non-smartphone plan. I also heard that even if you do you will only get "Edge".