Currently, US citizens re-entering the US via land (i.e. driving) are NOT
required to have a passport. However, the US government
and the Department of Homeland Security wants to change
that. The latest information is available at:
http://www.dhs.gov/xtrvlsec/crossingborders/whtibasics.shtm
As of Mar, 2008, here’s what the site had to say:
“For Land/Sea Travel
Beginning January 31, 2008, the United States ended the
practice of accepting oral declarations of citizenship at the
border.