From the
state department: Beginning
January 23, 2007, ALL persons, including U.S. citizens, traveling by
air between the United States and Canada, Mexico, Central and South America, the Caribbean, and Bermuda will be required to present a valid passport, Air NEXUS card, or U.S. Coast Guard Merchant Mariner Document, or an Alien Registration Card, Form I-551, if applicable.
As early as
January 1, 2008, ALL persons, including U.S. citizens, traveling between the U.S. and Canada, Mexico, Central and South America, the Caribbean, and Bermuda by
land or
sea (including ferries), may be required to present a valid passport or other documents as determined by the Department of Homeland Security. While recent legislative changes permit a later deadline, the Departments of State and Homeland Security are working to meet all requirements as soon as possible. Ample advance notice will be provided to enable the public to obtain passports or passport cards for land/sea entries.
The issue is volume when you drive I think. If you look at like the 405 or the 5 down past San Diego, the number of cars passing through per second is just too high. Its also somewhat usless to check, since you can go a few miles off the road and hop over the non existant fence if you want to get past. The air route is more of a target, and lower volume so easier to check.