Do you need a passport to travel in the USA?

First its not a good idea to piggyback your question on someone else's. This can lead to confusion. You should start a new thread. So I've moved your question to its own thread.

If you are a foreign national (other than Canadian or Mexican) then yes you need a passport to enter the US.

Do you need a passport to travel in the USA?
Yes

Once you get into the United States, you do not need a passport to travel from state to state, not generally speaking. But if you are going on an airplane from, for example, New York City to St. Louis, keep your passport handy, just in case.

Once you are inside the U.S. with a legal passport you do not need it to travel within; from State to State. However, with the present circumstances due to terrorism it would be prudent to keep it with you.
Stringer

To be more specific, if your from Canada and your driving into the US, NO you don't need a passport, if your flying, YES you do. Canadians driving into the US will need passports as of Summer 2008 but for now any government issued ID is good enough.

If your from any country other than Canada (or the US obviously) YES you need a passport