Idle observation, this: the growth in penetration of the Android base* by Ice Cream Sandwich in 2012 is tracking the progress of 2.3 in 2011, more or less.
(source is Google’s dev stats)
It takes a lot more time than people realise for an OEM to add a whole new version of an operating system to their carefully pieced together hardware platform and make sure everything works.
By the end of 2011, 2.3 had passed 50%. ICS will probably do the same.
(* Well, of devices connecting to Google play in the last fortnight, which is a can of worms)