Right now, you definitely make money if you are only using hardware that is already running, but not doing anything with its GPU.
Use this calculator to help you understand
For example, using my home PC, with a ATI 6770 card, I could get 180 Mh/s, and it uses about 75W extra power to do so, according to my little power meter. Given that, I would earn about $1.60 ib bitcoins per day (almost $50 a month) over my electricity costs. I don't do that, because I don't want to reduce the lifespan of my only video card, which is an important consideration, of course. edit: I have to wonder how warranty and product replacement plans would deal with cards burnt out by mining.
If you have a machine and/or GPU sitting around, you can check the expected hashrate on
these charts to see if it would be worthwhile.