Mining is not very profitable anymore unless you have an ASIC, which is a very expensive computer created specifically to mine them. Mining difficulty rises over time. When it started, you could mine with your CPU and make a profit. You can still do that now, but it's a waste - it uses far more electricity than you will gain in bitcoins. GPU is slightly better, just about break even iirc (like converting electricity into bitcoins). I'm running a java GPU miner right now (http://bitminter.com). As ASIC's get more popular and more common, GPU mining will enter obsolescence just like CPU mining did.
In other words, bitcoin mining has entered the big leagues.