The Sprint phone rebate is per line/phone. Once I bought 2 BB 9630 Tours and got $100 back on each.
To avoid Sprint rebates altogether, I recommend purchasing from Radio Shack. Been there done that too. Its easy. You just pay for the phone. No rebate required. Radio Shack is hooked up with Sprints database. You can buy new, upgrade, add line or whatever. For me it was a good experience. I bought my Evo from them.
If you buy from Radio Shack, I advise you go in person to your local store rather than doing it from the web. In person is simple. You plunk $50 and walk away with a RS $50 Gift Card. When you pick up the phone you plunk another $200 (and the $50 Gift Card) plus tax and you're done.