This question comes up reasonably often, and the best data is probably Sparkle stats, which is no doubt slightly skewed towards more tech-savvy users. Of course, this may be fine if that's your target market!
OmniGroup also provides stats from their updater: http://update.omnigroup.com/
The numbers are quite different to Sparkle's, and the graph of users over time indicates a slow decline for 10.4 (30%), a fast decline for 10.5 (25%), and a fast uptake of 10.6 (45%).
Finally, http://macosxhints.com has run a few polls recently about OS version and hardware type:
- http://www.macosxhints.com/polls/index.php?pid=snowingyet
- http://www.macosxhints.com/polls/index.php?pid=20100310053852122
Their numbers are similar to those from Sparkle, although they are self reported.
Most interestingly though, they show 45% of users who still own/use a PPC Mac, and just over 7% have Hackintoshes.