Finding out this information appears to be difficult - mostly you have to subscribe to a statistics gathering agency to get the figures.
Our figures indicate that MAC users make up less than 5% of the total web users.
We do support MAC, yes it is a pain, no it is not good monetary value for the effort that goes into it (sorry, MAC users!) but I can't imagine web sites disenfranchising MAC users, not unless you are preapred to lose 5% of your business and/or put up with the bad publicity
Suggest you implement a site management suite like WebTrends on your web servers, to (among other things) see what browsers and OS's your customers use. Our customers are schools, so Mac's make up much more than the 5% someone else quoted here.
I agree with Susan. But just an FYI the most common configuration is Windows 95/98 (basically the same OS with minor changes, basically 98 was a patch for 95) and I.E. 5.0 is the most popular browser worldwide. I forget where I heard that, but it was not to long ago.