I was looking at adage.com, and found the following article:
http://adage.com/digital/article?article_id=134209
ABC Says Web Viewers Will Tolerate Twice the Ads

Apparently research conducted by Nielsen shows that audience members viewing television shows online exhibit neither decreased recall or reduced ad attentiveness when showed more than one sponsor during ad break. It also suggests that the number of ads shown can be doubled without adverse effects... The article does acknowledge that there is a fine line between the right amount and too much-- which seems like a delicate balance to me!

So this is good news for the networks who distribute online-- but as someone who oftens streams shows, I have to say I am not so happy about these findings. Unlike a DVR, you cannot fast forward through the ad breaks-- but I do generally turn my computer on mute and get up during the breaks. It will be interesting to see if/which networks respond to this data.