Considering that Android activates 500,000 phones every day, it won't be long before the number of phones running ICS dwarfs any other version.
Iirc, Froyo took 8 months, and Gingerbread 12 months (just recently) to reach a 50% penetration of Android devices.
One thing that has changed about the relationship between Google and the hardware vendors is that Google now has it as a core objective to get the vendors onto the latest platform. In the past, I don't think that it was a high priority on Google's part.