At least one state (Florida) just passed a law requiring IDP for anyone with a foreign license--this includes Canada, oddly enough. It's worth keeping in mind.
Immigration and customs do not enforce insurance regulations. Police do, whether state or local. Most states require third party liability insurance--maybe all states do by now. In any case, you'll need it. Most buy from Progressive on line, with some stating a preference for (uh, let's make that Geico). You don't say anything about where you're entering (land border, ship, air), but I would strongly recommend not placing rubber on public roadways without first securing insurance.
Hope that's helpful.
Last edited by markharf; 5 Mar 2013 at 09:15.
Reason: sheer, unadulterated, foolishness