Here's an interesting ad I noticed on Yahoo Publisher Network (YPN) the other day. I didn't think these guys would run ads touting their direct competitors, but here it is, an Adsense ad displayed on YPN.
I can't say with certainty whether this ad was placed directly by Google, although I don't see a rationale for someone else paying to promote Adsense. I also wonder how it passed Yahoo's censors. But if Google indeed placed this ad and Yahoo didn't object, it shows an open-minded attitude from Yahoo. It's like CBS running commercials from ABC, or Time magazine publishing ads from Newsweek.
Online advertising works on a different level, I guess.