Do these new ASIC miners really pay for themselves in 5 days? The Bitforce 50GH/s miner costs $2,500 today, and according to the following calculation it would mine enough coins in about 5 days to pay for itself.

I am sure I am missing something here. Why would they even sell this hardware at all? Surely it makes more sense to keep the hardware and just set it loose on calculating coins and cash them in.
There's a factor called Difficulty, the more hashing power exists on the network the more difficulty exists, therefore it's harder to mine coins. Right now The difficulty factor would let you recover your money fast  because almost no one has ASIC chips, when they ship in bulk the difficulty factor will rise to 30-120 million during the year. To know by how much the difficulty will rise it's impossible, so it's your call.