Many of us living in border cities like San Diego are used to crossing the border to Mexico as a matter of routine. Upon entry all we had to do is state US Citizen, to the USCIS officer and he would let you in. This is all changing as of January 31, 2008. On January 31, DHS is ending this practice such that oral declarations alone to prove identity and citizenship will no longer be sufficient. Visitors will be asked to present certain documentation from a specified list when entering the United States at land or sea ports of entry.
Expect longer lines at the Ports of Entry.
Read the DHS press release here