2.6.<odd>: still a stable kernel, but accept bigger changes leading up to it (timeframe: a month or two).
2.<odd>.x: aim for big changes that may destabilize the kernel for several releases (timeframe: a year or two)
<odd>.x.x: Linus went crazy, broke absolutely everything, and rewrote the kernel to be a microkernel using a special message-passing version of Visual Basic. (timeframe: "we expect that he will be released from the mental institution in a decade or two").
Here's a list of devices that either have or will get Jelly Bean.
Also, this page shows a (fairly complete, though I hesitate to say "complete" for the fact that there's almost certainly at least one Android phone/tablet not mentioned on it that exists somewhere...) list of Android devices, including what version they run. It contains 41 mentions of 4.1 and 11 mentions of 4.2.
The difference here is that Windows has roughly 90% of the desktop marketshare (what is generally considered a monopoly market share), whereas iOS has an estimated 20%-50% (depending on who you ask) (30%-50% for Android).
Your code should be more efficient!