The warning here is important because thousands of American hunters have grown accustomed over the years to crossing the Canadian and Mexican border by car and coming back home with little paperwork. But those days are over.
To be sure, your failure to provide the above-mentioned documents through June 1 of 2009 will not get you in trouble. But it will delay you at the border until a Customs and Border Protection officer can verify your identity and citizenship. That could involve anything from a simple computer data query to a fingerprint check. After June 1 of 2009 you simply will not be able to transit the border at all without the documents outlined above. Your best bet, long term, is to get a passport. You can get more details on all this at the Department of Homeland Security.
Those traveling by air to Canada or Mexico, of course, must present a passport. That requirement has been in place for a while. The new border-crossing paperwork requirements are all part of meeting the requirements of the Western Hemisphere Travel Initiative designed to make US borders more secure.