Arron, I have just installed an add-in to IE7 called IE7Pro. One of the features is an "Ad Blocker". I am worried that it may cut into your impression count and therefore your ad revenue. If I right click and view properties on the blank area where the add should be, I do get a URL back to Google, so it implies they registered the impression.
And as you can see in the post, it also adds a spell checker!!