I just looked at the sales figures Apple reported. Apple has so far sold 95 million Macs that run the Mac App Store (Apple sells between 4 and 5 million Macs per quarter). The Mac App Store came preinstalled with Lion and was added to Snow Leopard in version 10.6.6.

If you are planning to offer your app on the Mac App Store, you don’t need to worry about compatibility with Leopard and Tiger, as those customers won’t see your app anyways. To me, that seems like a much better situation for developers than it is on the Windows side, where 40% of users still run XP.