I've noticed this on a number of pages, unfortunately including mine! In FF and Netscape (not IE or Opera), when you roll your mouse wheel when the mouse is positioned over an ad, the ad block itself moves up/down rather than the page. This happens with ad blocks, link thingos and referral buttons. Is there a known way to stop this? I know it doesn't happen on some sites.
It's because the advert is displayed in an IFRAME block, and some advertisers use slightly longer descriptions, causing the block to scroll, nothing you can do about it really, except in your mouse driver properties, you may have an option to set only the focused window to allow scroll, rather than whatever the cursor is over. Not a solution for your viewers, but it'll stop you being annoyed :P