I don't have a link, but you can probably find this. At DevCon there was a presentation on App World, with Alex Kinsella and two others, which had slides showing some interesting pie charts and such.
The gist of it was, that 5.x and 6.x are by far the bulk of the stuff out there, and if you look at who actually buys things on App World, those two plus the newer 7.x users are really the only ones who matter.
At this point, supporting any 4.x was deemed mostly pointless unless you clearly have a user base with phones that old, which only some organizations do.
The 5.x ones were, if I recall, still a good chunk of things, like 25%, so ignoring them cuts out a reasonably significant proportion of the potential user base, unless you're leveraging features that simply aren't on that version of the OS.
For any new app the minimum OS is 5.0, for some even 6.0
