Indeed! People look at the current calculator and say "13GH/s will net me 0.17BTC a day! This thing will pay itself off in less than 2 months!"
Eh, if it's 2 months without difficulty adjustments, it can't be that bad with, right?
I can't find a good calculator, almost all of them either want to factor in USD cost of hardware or do not have a difficulty increase field, which is really retarded.