Why would they? the OS is claimed to run Windows 7 apps, but no such claim has been made for Windows 7 drivers or Metro support, that I've seen yet, because that's out of Microsoft's hands. But that doesn't mean your apps will be compatible. I'm not taking any such claims of appcompat for granted since every new MS OS is a pain for driver support and BSODs until the nth service pack is released and software developers get on board with the new platform with optimized drivers.
Some don't even use Windows 7's new toys because the developers are stuck in ancient design methodologies. It is to laugh really, you have all kinds of driver conflicts and compatibility issues with each version of Windows, that's not [and never] going to change. If anything once Windows 8 matures appcompat will mature, but devs will just make native applications for it regardless.
What saves Windows is the software industries supporting Windows moving to that new platform, at least enough that the apps don't crash (and no even then) with updated versions of their old software, OR eventual support for multi-touch and Metro versions for Windows 8 (users have to have multi-touch input for there to be required support no?).. It is hoped that most of the stuff you need to use already are being converted to run on Windows 8 by launch and have support for Metro. Otherwise on x86, Windows 8, should as claimed by Microsoft run Windows 7 apps and drivers without any hiccups, but you'll still get and need new drivers versions when those updates ship.. Getting that same support for ARM will be >.> painful without software developers supporting it and enough devices to warrant that support.
And it is hoped that MS is hard at work on generic driver models to relieve some of that pain for ARM powered devices (and desktops) that need to run Windows 8 (hopefully set-top boxes, mobiles and consoles), maybe MS will show it when they have hardware ready. They were more interested in showing a functional x86 build (without any BSODs or the like) than Win8 preview for all hardware platforms (Phones Consoles and Servers) it will eventually support, and how it can be appropriate for each, and have something break on stage, versus something specific and flashy (Samsung Slate) that is ready for the market (Microsoft Store), and not some way off down the road and of little immediate interest to consumers.
You can check this out though: