Funny, some years ago prior to 9/11 I was traveling to Toronto Canada and as I approached Canadian Customs, I realized that I had forgotten my passport. They allowed me in based on my DL. However, I was concerned on way back to the States. Called my wife and asked to Fax my passport to our Toronto office. The faxed copy was accepted by US Customs. Big Sigh of relief.
Again, this was prior to 9/11 and don't know what would have happened in today's environment