I'm still rather new at the ads game, but I noticed an interesting phenomena on my own site a coupla months ago.
For a long time I had my google ads in the same position as the ones on datalk. I put them at the bottom and now they get more than twice as many clickthroughs. Go figure?
I had a hired gun tell me that top-of-page ads have become so commonplace that they're almost invisible; that folks are more likely to click the ones at the bottom while they're deciding where to go next.
Anywho.
Thought I'd pass that along!