What is the value of a brand-name in increasing click-thru rates for a
Pay-Per-Click ad?

I understand the legal issues of the use of brand-names in a PPC ad
(e.g. the Geico case), but I am trying to find any studies (or even
anecdotal data) about the improvement in click-thru rates for PPC ads
showing a recognizable brand name.  One example: we have run PPC ads
with a headline: "eBay software." However, eBay asked Google to pull
the use of "eBay" from all PPC ads, so we created ads with a headline:
"Auction Software".  The click-thru rates for the same keywords were
twice the rate when we used the word "eBay" in the headline.  This is
the kind of information I seek.