How common is dupuytren's contracture in the u.S.? Is there a lot of very mild cases that people don't know about?

3-6% incidence. Dupuytren's contracture affects 3-6% of the population of the United States. Cases vary btw very mild disease with nodules only, to severe cases with bilateral hand contractures, recurrence and involvement of the feet and/or penis. The disease is more common in people with northern european ancestry and has a strong family inheritance pattern.
Lower in US. Here also is a link to an eloquent article by a very well know hand surgeon from the pre internet fame days http://www.Ncbi.Nlm.Nih.Gov/pmc/articles/pmc1305903/.
3-4% of population. 3% to 4% of the population is the estimate.
3-6% 3-6 % is the most commonly quoted estimate. Higher in those with european heritage, especially northern europe.