Pic from Forbes.
Now, with Android 4, that could all change. Just based on what is coming from Google and some device makers, it does appear more devices will be eligible for Ice Cream Sandwich than anyone initially thought.
According to CNET, a Google rep told them that while there are no specific hardware requirements, most devices sold in the last couple of years should be fine. I'm sure that is the official statement that was released for the general public consumption.
However, projects like Cyanogenmod should be able to get ICS working on more devices because their elite effort has a way of stripping out stuff that are not necessary to the basic OS operations, giving life to older Android devices.
Only time will tell but I'm very optimistic...in the home brew guys. Not so much in the hardware guys. But we'll see.