I don't know much about the specifics of hardware needed to mine, but the best cards yield around 500-700 MH/s. Building a 5GH/s rig with those costs quite an awful bit of cash. To see that BTL sell that amount with much much less wattage and at only 300 bucks seems a bit fishy. They haven't shipped anything yet right?
Apparently to a very few chosen ones, lucky people, e-famous people, press and stuff.
Other than that I don't believe it's exactly a scam, but they're having trouble mass-producing them, they're also probably holding a good amount of ASICS for mining themselves, and with this waitlist nobody knows when it's going to reach you.