Factors that will boost online video advertising include an increase in broadband access, faster connections, the availability of more "premium" content and viewers' embrace of the medium's flexibility to pick what they watch and when, according to IDC.

Meanwhile, search advertising will continue as the single largest format for online advertising. Search advertising is currently dominated by Google with about 70 percent of the U.S. spending, IDC said.

Earlier this month, the Interactive Advertising Bureau (IAB) reported that U.S. online ad spending increased 26 percent in 2007 over 2006, and that the Google-dominated search format not only remained the market's largest, but also increased its share of the overall pie.

Search advertising accounted for 41 percent of the $21.2 billion in U.S. online spending last year, up from 40 percent in 2006, the IAB said. In 2007, spending in search advertising grew 30 percent over 2006.

Mobile Internet advertising hasn't taken off as had been expected, but it will also grow very fast, from a small base of about $50 million in 2007 to about $890 million in 2012, a compound annual growth rate of about 79 percent, Weide said.

With highly optimistic reports like these about the expected continued growth in online ad spending, the sense of urgency likely grows among companies like Microsoft, AOL and Yahoo, all of which have so far failed to capitalize as much their investors and executives have expected on online advertising's growth in recent years.