I've assumed half the reason Google's been on a custom logo binge is to drive users towards specific search terms and boost AdWords revenue. Show an obscure geeky thing one week to keep the baying nerd masses happy, show Michael Jackson the next. It's nice to see "the little guy" getting a boost as well!
what is really amazing is what amount of traffic goes directly to Wikipedia (just deducting from the spills that we are getting today).