I found an interesting post on installing the Surface SDK on top of Windows 7 (http://www.grumpydev.com/2009/05/17/surface-sdk-sp1-on-vista-and-win7-x64/). It requires a little MSI editing which you can do with the Orca MSI editor contained in the platform SDK. I'll post any additional gotchas I find as I test a few apps on top of the Windows 7 dev environment.