If you accept browser usage as a proxy for desktop deployments, you can find statistics pretty easily. http://www.w3counter.com/globalstats.php and http://www.w3schools.com/browsers/browsers_os.asp both show similar spreads between Linux, OS X, and Vista (eg, 2:5:15 and 4:6:17, as percent of total).

So web usage is comparable, but Vista seems to be well ahead. At the same time I suspect that deployments are quite a bit different than web usage, so take that with a grain of salt.