verizon 4s unlock to tmobile? is this do-able?

My dad uses a blackberry that's falling apart from all the wear and tear he's put on it over the past few years, so I was thinking about getting him an iPhone. I just saw a verizon 4s for a REALLY good price, but the thing is, I need it for tmobile. I've been reading online and the general consensus seems to be that it isn't possible, but there are always a few people who claim to have done it themselves. Can it be done? If so, how?