We've seen the rapid increase in mining power during recent weeks, and many mining result calculators still provide estimates based on a fixed difficulty which can be quite misleading.
Here's a mining calculator that takes difficulty increases into account (it calculates estimated daily difficulty change based on the last 10 actual difficulty changes):

    http://coinish.com/calc/

Try it, you'll be surprised by the results!

Onkel Paul

We're open for suggestions and improvements!