..."For a big campaign to work, they need 100 million unique impressions,'' he says."That's sort of a bar for it being interesting. There are plenty of people watching video. The challenge is where are they watching it. It isn't a lack of eyeballs but a lack of aggregated eyeballs."

Meanwhile, the relative scarcity of online video ad inventory has caused the cost per thousand impressions to climb about 15% to 20% this year, estimates James Kiernan, vice president and associate director of digital media and innovation at MediaVest USA in New York.

While a 30-second ad during a prime-time broadcast TV show typically fetches a CPM rate of about $20, a 15- or 30-second online video ad currently commands a CPM of around $20 to $50, Kiernan says.