According to the Outdoor Industry Foundation's recently published Outdoor Recreation Participation in the United States, paved-road cycling saw in increase of 1.7% from 2003 to 2004. They attribute this increase to (What else?) the Lance factor, noting that "Lance Armstrong has inspired a new generation of cyclists."
This seems to be good news for bicycling advocates, but "enthusiast" level partcipation is unchanged over the past seven years for road and mountain biking. Mountain biking in particular has seen a decline over the past couple of years among all participants. If you plot the points and draw a curvy line through the points, the Outdoor Industry Foundation notes that this rolling trend is flat for all types of cycling.
An interesting factoid to me is that road participation is relatively gender balanced, with 57% male and 43% female participants.
If you're a numbers geek like me, you might dig the facts and charts in the report. Download it as a 130 KB PDF file. I'll try to comment more on some of the numbers and trends as time allows.