One of the hottest topics in the online video world is whether ad dollars are shifting over from TV. Given that so much of original online video is free, and that ad spending as a whole is roughly flat, the success of online video pretty much depends on persuading media buyers to allocate some of their dollars to online video.

At last month's VideoNuze 2012 Online Video Advertising Summit, a panel of industry experts from GroupM, Digitas, Adap.tv, Collective and Canaan Partners discussed this topic and key challenges to increasing online video ad spending. Not surprisingly, the top challenge is improved viewer measurement and more specifically, unifying online metrics with traditional ones from TV. This wide-ranging discussion provides great insight into what media buyers are looking for and how the online video ad market is evolving.