One problem that I see is when a media buyer is trying to spend $10,000,000, it's a hell of a lot easier for them to cut a check to CBS and run TV spots. Because the online buy is still being viewed as somewhat experimental, the smaller buys become more work for the buyer.
A big hurdle in increasing online as a percentage of the overall media budget is the perception of value of an online view vs. a TV spot household. While volume #'s are low and more difficult to execute and analyze from a media buy perspective - the value of the engagement is higher on a number of fronts.
Viva la video.