It is happening for me too, but only in Firefox, and at a different height on the page.
In IE it is way down at the bottom of the screen, where it belongs.
I am running fully patched Windows XP, and the latest version of Firefox and Flash (the ad appears to be Flash).
I'm guessing that it has something to do with the extreme length of that thread's page, due to all the photos.
That add doesn't show up for me using firefox, I have the latest firefox, and all windows updates, I should have the newest flash (doesn't it force an update?)
Ummmmm.....okay, the only other thing I can think of is screen resolution.
On my 2 machines, I am running:
1024x768 - laptop
1280x1024 - desktop
I'd raise them higher but one is a laptop and the other is max res on a LCD.
I can say that on the desktop screen, the ad shows up further down the page. Maybe if you are running an even higher res, it manages to make it all the way to the bottom? (Just a total guess)