You certainly can. But it involves a lot of risk too, since there is no guarantee of those profits in the future.
In the peak of my GPU mining, I could make $100/day. Now, I am making $6/day, and more than half of that goes to pay the electricity for them. And that return will get even lower as more of these more efficient miners start being used.
Yup, some time last year I started CPU mining with about 16 Mhash/sec. I was making more per day than I make now with 340 Mh/s. My total capital investment to switch over to GPUs was zero as I already had ATI video cards in two boxes here which serve other purposes 7x24 and are not in any way dedicated to mining.
Obviously, I'm only a hobbyist. Mining has provided me with a few bitcoins to play with. I just point my miners right at a Bitcoinica account and mostly maintain a flat position in dollar terms by shorting an amount equal to my BTC balance, a condition which also earns a little swap along the way.