I guess I'll add my .02 to this. I was on the phone with a very knowledgeable Sprint rep (wait, whaa?). I know I know, doesn't seem logical.
But she was actually very competent of everything going on. I was actually on the phone for something else but decided to ask her if it would be possible to activate a Verizon phone on Sprint. What she said was this, anyone at telesales or online absolutely would NOT do this for you. They wouldn't get in trouble, but the way she explained it to me, that person would be liable for that phone getting activated on Sprint.
So, what she meant by that is, that if your phone would break (Verizon phone say, Pre 3) then they would have to replace it with a comparable model and they don't want to do that.
But then, she gave me this little tid bit of information, the only chance of having it activated would be in stores. Maybe if you have a friend or something, she said all the person would need to do is type in the serial number or whatever it is on the back, I forget what term she used, and then the phone would be active.
Then she stated, that person would be responsible blah blah, like she said before.
So, it's definitely possible, but good luck finding that person in store who would be willing to do it for you.