I've read via Phoronix the case of the indie PC game producer Koonsolo, which sells a game for both Windows, Mac and Linux. The interesting thing is that, as you can read on Koonsolo's blog, the Linux version is being sold in larger numbers than the Windows one!
Apparently a 40% of the visitors or the Koonsolo site use Windows, vs less than 23% for Linux. However, despite the majority of visitors using Windows (there are even more Mac visitors than Linux ones), the Linux version sales amount to a 34% of the total sales, whereas Windows sales are only 23%. Visit the site for some more numbers and comments.