Reported at ClickZ (but for some reason not widely reported by traditional media), the article states that,
The research company conducted over 14,000 surveys evaluating 238 brands, 412 products, and 951 ad executions, and collected data on general recall, brand recall, message recall, and likeability. The results suggest that for each metric, consumers reacted better to ads delivered via online video than they did through traditional TV.
Nielsen says the increased impact could be attributed to the nature of the viewing experiences offered between the two platforms, with online video viewers often more "engaged and attentive" to the content they are consuming.
This wasn't a small impact either - online ads were on average more than 30% more effective per the chart below.
To learn more about how people are watching video, I recommend reading Nielsen's report, How People Watch: A Global Nielsen Consumer Report.