Online Mining Calculators using prebuilt machines data
The problem is that the difficulty rating is going up, and going up significantly.  This happens for two reasons.  First, the number of mining rigs increases as people try to get involved with Bitcoin mining.  Second, ASIC-based miners which can run over 60GH/s using around 600W are just arriving in the market and once more generally available will cause a big leap in the difficulty.