Emily wrote:
I dunno, last I checked, you weren't required to carry ID in the
United States.

Well - if a police officer has "reasonable suspicion"[1] that you committed
a crime and the jurisdiction has a "stop and identify" statute[2] then you
must present identification or face arrest. That providing identification
is rarely relevant to establishing whether a person has in fact committed a
crime appears itself to be irrelevant as judged by the Supreme Court of the
U.S.[3] Go figure.

There are "stop and identify" statutes in many states, and the
reference in [2] below has a section titled "How to satisfy the minimum
required duties" that provides some suggestions on how one might maximize
ones rights if faced with such a situation.

[1] http://en.wikipedia.org/wiki/Reasonable_suspicion
[2] http://en.wikipedia.org/wiki/Stop_and_Identify_statutes
[3] http://en.wikipedia.org/wiki/Hiibel_...ourt_of_Nevada