Like others have said, I suppose you will be able to run Vista 32- or 64-bit on current notebook hardware, just not very well, especially not the eye candy features of AeroGlass and eventually Avalon (and let's face it, who doesn't want the eye candy?). I think the primary limiting factors would be graphics hardware (remember that Intel still owns the graphics market by a comfortable margin) and memory, again, like others have mentioned. It would be like trying to run XP SP2 on "designed for" Windows 98/ME/2000 hardware.
What I'm interested in seeing is whether or not some of the newer technology that Vista will support, like cable cards, HD-DVD and Blu-Ray DVD drives, hybrid HDD's, and onboard NVRAM cache, not to mention some of the less attractive technologies like DRM enabled multimedia software requiring HDCP compliant monitors, will have retrospective support under Windows XP or if you'll just be forced to upgrade.