If it sounds a lot like AT&T's recently unveiled promotion, that's because it is. In fact, AT&T's move was seen as a pre-emptive strike against this move by rival T-Mo, as it was an open secret that T-Mobile was going to offer this promotion.
Although the ad doesn't give details, and clicking on it results in being taken to a dead page, the URL (http://www.t-mobile.com/offer/switch-carriers-no-early-termination-fee.html?AID=11031750&PID=6150529) and the ad itself give some clues. The fine print of the ad, for example, says:
Qual’g service & device purchase with port-in from AT&T, Verizon, or Sprint req’d. Up to 5 lines.This differs from AT&T's promotion which only applies to T-Mo switchers.
While still unclear until the actual reveal, it's believed that T-Mobile will offer customers $350 per line to pay ETF fees. AT&T's promotion offers $450 per line, but due to limitations and other gotchas, it's expected that most will not be able to achieve that number.
Device purchase is required, but right now customers can opt-in to a Samsung device with no down payment. T-Mo's UnCarrier program eliminates carrier subsidies by having customers pay a down payment on the device and then pay monthly until the device cost is fully accounted for.
The new measures have been helping T-Mobile gain market share. The latest Kantar report on smartphones showed that -- in terms of sales in Kantar's 12-week window -- the nation's fourth largest carrier has moved into third, ahead of Sprint, and that it is the only major carrier to show year-over-year growth.
Update: The numbers have been confirmed. T-Mobile says that with an eligible phone trade-in, new customers can get up to $650 per line, and that includes up to $350 per line for ETFs.