Online advertising - Google uses CPM

Hot on the heels of recent comments that on-line advertising has not found a suitable alternative to the "cost-per-click" business model (known as CPC by those in the trade), Google is launching a new service based on the "cost per mille" model (known as CPM by those in the trade). The cost-per-mille model means advertisers pay a fee for every thousand people. Why is Google doing this? What impact will it have, if any? Will this bring a wholesale change to the on-line advertising business models?

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