Apple has not always supported two versions back. It has usually been the case that the latest version of Xcode requires the latest version of OS X. See Xcode 2.0, Xcode 3.0, Xcode 3.2, Xcode 4.0. Xcode 4.2 was an aberration.

As far as my stats go, around 85% of programmers that use Macs are using Lion. I imagine for Mac and iOS developers specifically it's well over 90%.