Just taking a guess here, but I'd imagine that everyone's CTR would increase by having a horizontal navigation bar with four or five "link unit" ads (vs. "ad unit"). Set a background image for the ad space that makes the links appear as tabs - users think its navigation for the site and would be more likely to click(?). The first tab would still be the "Ads by Google" tab though.
Has anyone tested this? Care to share the results?
I'd imagine that this doesn't comply with Google's TOS - so it may not be worth the risk...
Ad and image placement: a policy clarification
I used to use a wordpress theme that had such a link unit. the ctr was decent and it complied with the adsense tos(dunno if it still does). Still,I imagine that the big adsense blocks will still get a higher ctr than the link unit but since the link unit will be at the top of the site it will have the highest paying ads so not really worth it imo. I could be wrong tho, didn't test this with arbi yet.