They're so annoying, at least in Vista you could disable them! There's a Group Policy item for it, but it doesn't do anything. From what I read it only worked pre-RTM.
well, the thumbnails are much more useful in win7, than vista. Disabling them is a bad idea, but you can always switch to "basic" theme
Eww. I shouldn't have to disable Aero entirely just to disable one aspect of it! Especially when it was an option in Vista. I use the "Never Combine" taskbar option, which means that I have labels for each window and there is no grouping, which makes thumbnails useless and distracting. :|