How do we know? Well, the strongest clue that we came across is the fact that the handset is listed with its SGH-T379 model name, which gets pretty close to the Samsung t379 Triumph that was mentioned in a leaked T-Mobile roadmap not long ago. In case if you are wondering, the device in the filing does support T-Mobile's frequencies and can even run on the carrier's 3G pipelines at theoretical speeds of up to 7.2 Mbps. Some of the other goodies that it sports are a side-sliding QWERTY keyboard, 2-megapixel camera, Bluetooth and GPS, though a Wi-Fi radio is sadly missing.
The Samsung Triumph, or whatever its real market name turns out to be, is believed to be a successor to the Samsung Gravity 3 feature phone, and it wouldn't be surprising if we see it sitting on T-Mobile's shelves pretty soon.