Well, gee. While we're on that, which states have I been to? Lessee...
Washington, Oregon, California, Nevada, Arizona, Utah, Idaho, Montana, Wyoming, Colorado, Nebraska. When I was six we drove to Georgia, which means I must have at least driven through Missouri, Illinois (I remember we stopped to visit one of dad's old college buddies there), Kentucky, and Tennessee. And perhaps either Kansas or Iowa. I'll have to check with my parents. And then I've been to Florida, Georgia, South Carolina, North Carolina, Virginia, (Washington D.C.), Maryland, and Pennsylvania.
Lessee, that's about 23 out of 50. Hmm. My Northeast experience is definitely lacking (it consists of one day in Pennsylvania when I was twelve), and my Midwest experience is quite superficial (we drove through it when I was six). Besides New Mexico, I've definitely got the West down pat, and I've done alright with the Southeast, too.
June 13, 2007 edit: Add Massachusetts, Connecticut (passed through in bus), New York (2-hour stopover in NYC), New Jersey (bus), Delaware (stopped off in train to get a picture of me in Delaware), Alabama (bus), Mississippi (bus), Louisiana (bus), Texas, and New Mexico (bus- and an interesting bus it was). That rounds out the West and the South pretty well. Still have some work to do in the Midwest and Northeast, though. Current total: 33/50.
September 5, 2013 edit: I've added Michigan at some point in the last six years. 34/50.
April 18, 2014 edit: Add Oklahoma, Indiana, and Ohio, from my drive from Austin to Boston. (Also, I now think it was Kansas and not Iowa that we drove through at age six, since I specifically remember being on guard for tornadoes à la The Wizard of Oz and being disappointed that I didn't see any.) 37/50.
July 8th, 2014 edit: Add Vermont and New Hampshire, from our weekend trip. 39/50