Keep in mind that Google is changing not just the capability of the OS and how it deals with software/hardware, but the user interface as well. The rumors are that Ice Cream Sandwich is going to use software buttons like Honeycomb so I would expect larger screens to accommodate these buttons that we are familiar with but layered on the screen.
I am assuming that we are going to see more 4.3-4.5" screens that are edge to edge with the bezel, in addition to more streamlined body designs due to the lack of physical buttons. For all of us that root and mod our phones I am sure that they will keep the power button and volume rocker so we can use the likes of clockworkmod and other recovery utilities.
But in addition to all of that I believe they are going to try to take a fresh approach to notifications and the notification bar. Instead of a drop down screen we could see some type of holographic or transparent overlay that can be pulled up with a swype gesture or by touching a software button that is visible from all programs.
How they will tackle this in regards to full-screen games/content I am unsure, but I believe they will have to put certain standards and protocols in place as to where to access menus and such so that users don't have the same confusion that they have with Honeycomb.
As far as hardware support Google has already rolled out app compatibility support on the official market so if they offer the upgrade or if there is a port of the OS from ROM developers you should be able to tell if you are going to be screwed or not. But keep in mind that Android years are shorter than dog years and a two year contract is a long time to wait for an upgrade. I would look at memory (RAM) as the most important buying decision in future-proofing myself to get ICS as everything including buttons are software driven and live in the momory...