Windows 7 is just warmed over Fista. That's it, the changes in the code base are trivial. However, there is a new feature, VXP, or Virtual XP. It basically sets aside part of the computer's memory to run a Windows XP emulator for programs not compatible with Fista/7, providing desktop links and everything. As one of the Grokbots put it, "This basically means that Windows is incompatible with Windows."
What does this mean for developers? I don't imagine much, since M$ is still trying to force-phase out XP in favor of the new code base in Fista, so this is probably to keep people with legacy software from jumping to Linux and running under Wine. However, here's the question:
Since it will take system resources to basically run two OS'es, why not just revert to XP and run a single OS? Oh...because then, you won't have an excuse to buy a whole new Win OS and shell out for new hardware just to run it.
They say, "Never attribute to malice what can be explained by stupidity." I would like to point out that those terms are not mutually exclusive.