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Up vote just for use of the word "penetration"

As weird as it sounds... it's the proper term (yes, really!).

Adobe also did it:
http://www.adobe.com/products/player_census/flashplayer/version_penetration.html

This is useful for general web projects, but a lot of Java server development is for enterprise environments, no? It would be interesting to see what the desired/required Java version is for those projects. I suspect that 1.6 penetration for those types of projects is *much* lower and even 1.5 penetration isn't that high (at least in my experience).

Enterprise is completely different. There you either have the freedom to choose or the requirement to be compatible to a specific version. In some cases it's as bad as 1.3.

Creating a semi-meaningful statistic for that appears to be pretty difficult. I guess you would need some kind of massive survey.