Michael DeGusta has done a lot of work:
I went back and found every Android phone shipped in the United States up through the middle of last year. I then tracked down every update that was released for each device - be it a major OS upgrade or a minor support patch - as well as prices and release & discontinuation dates. I compared these dates & versions to the currently shipping version of Android at the time. The resulting picture isn’t pretty - well, not for Android users.
Look at that graph.
It’s sad for Android users, really. The Nexus One actually had the best update record by far, and it was just cut off less than two years after its release.