Know any examples?
Perhaps there might be some widgets that don't run everywhere ? Or perhaps an ICS specific tablet app?
Overall, I can't think of any major Android apps that I've run across that I couldn't run on every device, at least not since the early days of the Netflix app or some games a year or two ago.
Just as with iOS, most Android apps wouldn't need anything past the basic APIs that have been around for a while and which now make up the majority of ownership.
The fragmentation boogy-man is way overblown, IMO.