NewTeeVee has a nice post summarizing some recent Nielsen data on TV viewing + simultaneous Internet usage.
Of course, the data are somewhat anecdotal because Nielsen has direct measurement tools for TV viewing, but noting “Internet time” is diary-style and the diary is likely filled out by older household members less likely to multi-media-task™ with laptop and remote on the couch.
It also ignores the younger set that doesn’t turn on the TV to begin with, but still watches TV content… via the web. I’m working with some new folks in St. Louis, some rather young folks, and they generally don’t watch TV via TV. So that usage pattern isn’t directly recorded, either.
Nevertheless, the dual-surfing approach is definitely on the rise, by Nielsen’s numbers and from my own observations. Shoot — my 70+ year old mother does that with her MacBook Pro and the big screen TV. Soon she’ll do it with her 64GB WiFi iPad (I kid you not).
Gravity Medium 
Robin is getting an iPad too for all these reasons