Vibrant was the SGH-T959, and the Galaxy S 4G was the SGH-T959V -- so you can imagine that this is another device in the same vein. Samsung SGH product codes that start with "T" and end with "9" are T-Mobile devices... but if this is for T-Mobile, where's the AWS 3G support?
So here's our wild theory: T-Mobile and AT&T may have fast-tracked a T-Mobile-branded device -- in this case, an offshoot of the Galaxy S 4G -- with HSPA+ for AT&T's bands. AT&T has mentioned this week that one of the first fruits of its planned acquisition of T-Mobile USA will be a roaming agreement that allows T-Mobile customers to use AT&T's network, and naturally, that would require devices that support AT&T's spectrum; interestingly, this roaming agreement is going into effect whether the acquisition completes or not, so these guys have to deal with it. We'd doubt that either of these carriers would be keen on releasing yet another variant of the original Galaxy S as we move toward mid-2011 here, but it's entirely possible that T-Mobile and AT&T employees could be getting these to test the network integration over the course of the year. Crazy, yes... but just crazy enough to be plausible.
Update: Here's another thought. This could be an HSPA+ device (a la Galaxy S 4G) for a Canadian network, since several of those guys have also deployed Samsung devices that start with "T" and end with "9" -- the model number exclusivity to T-Mobile is strictly within American borders. We'll find out soon enough!