OKAY. With a Butterfly Labs 50GH/s miner, I can actually make about $40 a day, by just hooking it to my laptop, mining, and selling my BTC on Mt. Gox.
WTF IS THE CATCH!???
Simple, you are going to make about $40/day for some 10 days or so, then around $34/day for another 10 days or so, then like $29/day etc ...
The sum of that income until the miner breaks down or earns less than your electricty bill probably wont exceed the cost for the miner.
Rising prices for BTC, fairly low difficulty increases, getting a miner really cheap or other factors
might let you actually earn more than you spend, but that is far from certain.