bcc1234 - 3:55 am on Jun 12, 2010 (gmt 0)

but it wasn't Google, that's the only thing I'm sure of.

I wouldn't be so sure. It might not be AdSense, but third party networks ads are served from other domains. So you would see something like ad.somedomain.com, not googlesyndication or doubleclick. The ad block frame entirely redirects to a third party domain.

I'm having the same problem:

[webmasterworld.com...]

And I don't show anything other than Adsense (with image ads) and in-house (hardcoded) ads.