shorebreak - 6:26 pm on Sep 6, 2010 (gmt 0)
Looking at the data, a couple datapoints worth noting:
7% of spenders account for 60% of spend
26% of spenders account for 84% of spend
It's too bad there's no historical trend data in the document AdAge got their hands on. I'd bet that if there were, though, you'd only see a slight diversification over time away from reliance on these large advertisers for the majority of Google's revenue.