Yes. As of June 1st 09, you need either a passport, an enhanced drivers ID, or one of the few other things listed here.
http://www.getyouhome.gov/html/lang_eng/index.html
This is the first year that you're going to be REQUIRED to have one of these types of ID. MAKE SURE YOU GET ON THIS NOW, GUYS & GALS. It took a month and change to get my passport back.
Also BE AWARE that the Canadians MAY let you in without having a passport (not sure, WHTI has probably made them comparable to the US crossing), but you ABSOLUTELY will not get back into the US without it, leaving you stuck at the American Embassy in Canada for a couple days to sort shit out.