Adium (the open source chat client) publishes their Sparkle update data. This may be a good approximation of the market share of each Mac OS X version, as well as CPU types and Mac model. Just proportionate the percentages with the total number of Mac OS X users (about 9% of personal computer users) and you should be able to get a fairly good ballpark figure of the number of users of each OS.
Of course if Google publish their home page's browser-agent statistics, it will be a more accurate estimate. But too bad that isn't happening (anybody at Google reading this? ;-) )
I hope this helps.
