Check out this video. Although this is an extreme example of what a national ID card can do, there are probably some elements of truth. Marketers today compile tons of info on all of us. A national ID card could be the missing like that allows corporations and the government to do even more detailed tracking.
My brother set up a link on my Mozilla browser called "Stumble!". It takes info I provide and spits out links such as this video any time I hit the button. I can vote on its quality and it uses that data to recommend other links. The conspiracy theorist in me thinks there's some sort of marketing use. Coincidentally, I did tell the folks at "Stumble!" that I'm interested in Conspiracy Theory and Civil Liberties...they then probably forwarded my name to the FBI or DHS, to put me on "The List".