You do have to attach the peripheral though from what i read so it's not like you wouldn't know it's happening. Though I'm sure at some point the hardware will be built in by default and set to always-on just for your convenience <img src="smileys/smiley2.gif" border="0" align="middle" />
If you can also track pupil dilation though it could be useful to gauge emotional response. That and seeing what you're looking at and in what order would definitely be useful to advertisers.
O_o
Whoa.... that would be awesome.
Although I suppose there would be security issues with a website tracking your EYE MOVEMENTS!!