Here's the deals I can see. I'm currently on a $60/mo no contract plan.
If I buy at the t-mobile store this weekend, I pay for the S3 $150 and then have to send in for the rebate, so I basically have tax of $10.50. I also can go on a value plan for $59.99 unlimited talk/text/2G data. Add to that the subsidy of $20/mo over 20 months($80x20) = $1600.
If I buy it for $150, at say, Walmart, have to go to the classic plan, Which is $69.99/mo, that would be $70x20 = $1400 + $150 = $1550? I don't see the big savings. And buying at the t-mobile store this weekend enables me to keep my same number.