I just thought I would chime in here since I plan on taking advantage of this and may be in the same boat as some of you.
I have 4 lines with AT&T.
I have the following phones:
iPhone 5S
iPhone 4S
Blackberry Curve
Pantech Ease
3 of the lines have early termination totally around $700
Obviously the Blackberry and the Pantech are not worth jack, but the 2 iPhones are easily worth $800. T-Mobile would give me $400 for the two, so it would obviously be insane for me to do that.
I went into my local Corporate T-Mobile store and confirmed that you don't have to trade in your current phone, but for every line that has an ETF, you have to turn in a phone that is on their list. ANY phone on their list as long as it powers on and doesn't have a broken screen.
You can figure out which phones are on their approved list by going to their site and checking the trade in value of old phones:
http://www.t-mobile.com/DeviceRecovery/cell-phone-trade-in.aspx
The cheapest phone I could find on their list is an AT&T Go Phone ZTE Z431
You can buy them new on Amazon for $35 or better yet, I see them used on eBay for as little as $9
I'm still not sure if the Blackberry is eligible to trade in yet because my son-in-law is out and about and I can't reach him to get the model number, but even if I have to buy 3 Go Phones, that will cost me maybe $30. T-Mobile pays off my $700 ETFs and I get 4 new iPhones for 0 down and I sell the 2 old iPhones for $800. To top it off, I'm going to be saving at least $80 a month going to AT&T and probably even more because they have no overages and no need to get an international data plan for my son in law every time he goes to Spain for 3 months a year.
There's nothing underhanded or unethical about any of this. I told 3 different sales people at the store, including the manager, what I was going to do, and they all said it was fine.
After working all the details of this deal out, I would be insane NOT to do this. Everybody in my family gets new iPhones and I profit by around $750 and save $80+ a month and get better service.
Actually, I may even just but the cheapest phone they have on a payment plan and just put the SIM in my current iPhones since they will work on T-Mobile's network. This is getting more interesting all the time.