MasonM wrote an excellent piece on the RealID legislation submitted in the most recent defense appropriations bill. While his argument was sound, it addressed the concern that we would soon have to show our papers just to travel in the country.
The sad truth is, this is already the case.
17 years ago I learned this truth in Tacoma, Washington. In most municipalities, if you are wandering the streets, you can be pulled over and asked for ID. If you don't provide it, you can be detained until your identity is determined. If, in addition, you have no money or proof of employment on your person, you can be busted and held for vagrancy.
If you travel on virtually any form of "public" intercity transportation in this country, you must present ID. Don't believe me? Try it! In addition, you can be randomly stopped on said intercity transportation and asked to provide "papers" before disembarking (as we learned in Albuquerque, NM, when we couldn't leave the bus until we had shown ID).
The concerns I have about the RealID program are that it will provide a federal database for ALL citizens possessing said ID (such a database previously only existed for convicted criminals). It holds further, more sinister potential for abuse, but for the sake of this argument, I will cede to the government the purest of motives.
But the realities of showing "papers" to travel are already here. Whether we agree or disagree with them, we must do so on the knowledge that THAT future, at least, is already upon us.