I just read the Vista licensing agreement, and saw no such clause preventing a downgrade (for home users anyways). Even if such a clause did exist, it cannot POSSIBLY be legally binding for 2 reasons:
1- You own the hardware. Period. MS has NO legal authority over the device itself to tell you what you can and cannot do with such a device.
2- If you own a prior legal version of Windows, such as XP, and meet it's licensing terms, then you can legally install it. I haven't seen any language in the XP agreement that claims I will be unable to reinstall it if I upgrade to a future version...
I have seen and heard all kinds of posts all over the web about this, with some people even believing it will ruin their computer warranty. To date I haven't seen any such language in a warranty, etc. Ignore the rumors.