I've seen that, but I'm curious to know if it lines up with the real world. Are you making 2BTC/day from 500mh/s?
It lines up perfectly. Bitcoin is pure math. You can take just about any handful of numbers related to mining and accurately predict the rest.
My 570 Mhash box with a 6970 and 5770 that draws 445W at the wall (gaming box with multiple HDDs, fans etc) creates about 2.35 BTC per day, just like the calc says.
Of course, there are going to be small variances based on changing pool membership, software hangs, and things of that sort. I've observed the calculator to be accurate within 3% day to day.