You can, of course, wait for Windows Vista to ship before checking out the property system. But if you are curious or bored, here are a few other things you might do:
On a more sober note, if you build a project using the Vista PSDK and plan to run the apps on XP, be sure to ask for the Windows XP/2003 subsystem and header macros. If not, your app will assume all those nifty Vista APIs are on your system. Unfortunately, I don't know how to do this myself yet, but if I figure this out, I'll mention it here.
[2006/09/07 - BK: I've been told this is a stable link to current and future Windows Vista Beta downloads, including its SDKs and RC1 content: http://msdn.microsoft.com/windowsvista/downloads/products/getthebeta/]