The prerequisites could be handled in the same way that WP7 handles them already, by warning the user at install time in the store.

For example, some apps require a gyroscope, which I think handsets only came out with in the second generation. If you try to install one of these apps in the store, you'll be warned that some functionality won't work (or perhaps won't work at all - I haven't seen this). Similarly, I've seen reports that in the upcoming update aimed at lower end handsets, there'll be warnings in the store for the few apps that don't work with limited ram requirements.

I would predict Windows 8 will keep a similar list of the device's capabilities and warn the user in the store if they try to buy something that won't work.