Here is estimates on earnings when some of the ASICs hits the marked. Note that those after this date are estimates, not proof.
However take a look at the previous records on mining. They is 100% real and how Bitmining have changed and it gives a great picture on why GPU mining is dead.
2013-02-01: 0.169 BTC per day with 1GH/s. Back then you made 5.07 BTC/month (0.169*30days).
2013-04-18: 0.056 BTC per day with 1GH/s. That is today. So in a month now you make 1.68 BTC/month (0.056*30).
So without ASICs flooding the market from Avalon/BFL, due to the difficulty you now mine 65% less Bitcoins/month than 2 months ago. Just look at the chart, if his estimation on ASICs hitting the market is correct, you will need several ASICs to make a decent slump of money.
You can find terms about the difficulty and earnings after this date is based on here:
http://organofcorti.blogspot.no/2013/04/913-asic-choices-asic-earnings-17-april.html