If you mean a TRUE mac...it's not possible unless you buy old gutted macs and parts from them. I mean I've built some OLD macs before taking the parts from one broken mac and sticking them in another.
If you mean just general parts and installing OSX on it, I don't think they have a very good release of OSX for PC just yet. APparently it's possible but I don't know how sucessful people have been with it yet.
The version of OS X currently on release is an absolute nightwamre to install. It takes about 5 hours to get up and running and WILL NOT run very well on even the fastest of PCs.
I agree with The Other One that building a mac from mac parts is pretty much not worth it. We do it every day for clients and although you'd save a hundred bucks, you'd still need motherboard and processor as well as a decent video card. Of course, there are third party companies out there that do this for a living, so there must be some form of saving otherwise there wouldn't be a profit.