Apparently, historically, it USED TO be 2" x 4", but over time, the standards changed.
Actually, historically the "2x4" came from the original rough cut "green" lumber (freshly cut down), which then was dried and planed and ended up at something close to the 1.5" x 3.5" mark. In the early part of the 20th century, finished dimensional lumber standards were introduced which codified the "final" acceptable measure. Subsequent to that, process improvements led to the lumber mills being able to produce something closer to the finished standard as a rough cut, so now they don't cut 2 x 4 to start, but the finished product is nearly the same. I believe Gumbo mentioned 1 5/8ths x 3 5/8ths. That was the standard up until about the 1960's, if memory of my recent reading serves.
I started looking into this when I first read the article out of morbid curiosity. I always knew 1 1/2 x 3 1/2, having framed in basements and done a few other projects around the house over the years. I didn't know the history of how it got there.
Incidentally, hardwoods have a different standard and the actual measures are closer to the nominal measures. 1 3/4" x 3 3/4" for a hardwood 2 x 4.