I don't see why it's not legit. I've seen that kind of offer before for Sidekick. The 300-50 is the original offer from T-mobile and I've seen many other places offering better deal.
Honestly, I regret buying the Sidekick. It's slow as hell and not convinient to dial phone calls. I basically only used it for the first 2 months and it's been sitting in my drawer.
Interested in purchasing a Sidkick II. T-Mobile is offering it currently for $300 less a $50 rebate=$250. Found an offer on the 'net for Virgin Megastores, offering the phone for $100-$100 rebate=FREE. Has anyone dealt with Virgin??? Please share any information as of course, this seems TOO GOOD TO BE TRUE!