This is very cool, and when I read Tim Heuer's post, I actually had to read it twice, because I didn't think I read it correctly the first time!
It's read-only, and under a Microsoft reference license, but that's better than a poke in the eye with a sharp stick.
Even better than that, IMHO, is that with VS2008 we'll be able to debug down into the framework the way we have been able to into win32 SDK. I feel like I've spent half my life staring at win32 SDK code in the debugger, so begin able to debug down into the framework is seriously cool.
Scott Guthrie blogged this as well.
Read the two posts for the particulars about all this.