It's only valid for pages built with FBML. So it's good for tabs (sections) but not for the main page itself. The main page is a newsfeed where you just can't put any FBML. So it makes an interesting tool to track subpages but in general it's pretty useless. Clever idea though.
I wonder if this might lead Facebook to start caching images on fan pages like they sometimes do for images in apps.
This covers fan pages - anybody know how to do the same for personal pages, in case you feel like snooping on people who are looking you up?
But then I see this:
http://grownupgeek.com/facebook-forums-whos-been-looking-at-...
Apparently it's against the TOS.