As a previous poster noted, the Supreme Court determined that the Police, in the course of an investigation, can ask a person to identify herself (joke: pull out a pocket mirror, look at it and say "Yep, that's me!). A "official" ID isn't mandated and the person could just state his name and home address or possibly name and birthday. If the investigation involves something that requires "official" ID (driving, hunting, buying alcohol, etc) the cops can demand to see the person's "official" ID.
Oddly, many people believe that an ID is needed to take a commercial flight but that isn't the case. You will just get to go through "enhanced" security (wand, pat-down).
However, the same isn't true for Amtrack (from
http://www.amtrak.com/passenger-identification):
Passenger Identification
Amtrak has undertaken heightened security measures for the benefit of our customers.
Valid Photo Identification Required
Photo ID Required
Amtrak customers 18 years of age and older must produce valid photo identification when:
Obtaining, exchanging or refunding tickets
Storing baggage at stations
Checking baggage
Sending Amtrak Express shipments
Onboard trains, in response to a request by an Amtrak employee
Please note that unaccompanied children 16 - 17 must also produce valid photo ID when purchasing tickets, obtaining travel documents and checking baggage.
Random Ticket/ID Checks
Following federal Transportation Security Administration (TSA) guidelines, we regularly conduct random ticket verification checks onboard trains to ensure that passengers are properly ticketed. Please be prepared to show valid photo identification to a member of the onboard crew upon request.
What is a Valid ID?
To be valid, your identification must be current and in-force. The following forms of identification are acceptable for persons 18 and older:
One piece of photo identification issued by a government authority, or
Two pieces of identification, at least one of which is issued by a government authority