From what I'm reading it appears to be a side effect of enhanced security in IE7. Google lead me to a thread on Ars Technica where they're discussing the same thing. There's some Grandma-unfriendly language there, so I'm not gonna provide a link.
What I gathered from the discussion though is that IE7 is sometimes (but not always) asking users to verify usage of plugins they've already installed. It also seems to be asking on websites which don't obviously seem to be offering content related to the plugin being verified.
Poking around some more I came across this link which discusses the problem in a some degree of technical detail and provides suggested server side fixes.