I actually spoke with a rep at my local Morris store about this. I asked him directly if I could port my number from T-mob, get an Evo 4G now and then exchange it after the E3D arrives and still retain the credit. He said yes. The port in credit is not dependant on the purchased handset. He did say there would be a re-stocking credit of $35, but that still puts me ahead $90. He also recommended picking up the phone of my choice only when that store started pre-orders. That way, there would only be two weeks until the phone was released. Basically, he talked me out of bringing my number over today so I could get what I wanted. Good customer service there
The end result for Sprint is a net gain of one customer on a contract. That tends to be more valuable over the length of the contract than just the subsidized phone. Or at least that is my understanding of it.