It is my understanding that the current reward for solving a block is 25 BTC. Is this calculator simply dividing that reward by the number of days/weeks/months expected to solve a block and assigning a BTC value to it? If so, this can be misleading to those uninitiated, as it makes it seem as if they will be getting a constant reward of ~0.188 BTC per month instead of one lump sum of 25 BTC + transaction fees.
@Scatman, why is the hashrate in your image at 7.5 GH/s while the miner you are selling clocks at 5 GH/s?
Also, keep in mind that the mtgox price is inflated because people have a hard time getting their fiat money out.
I have adjusted to the proper values and the current market price on bitstamp and got this:
I personally don't think it is worth it, especially that you are not factoring in the price of electricity and the depreciation of the miner itself, which will be obsolete as the total power of the network is increasing.