NuMD97,
There are some U.S. 4th year medical students that have already matched. There are a couple of ways this is possible. First, you could have already matched through the military match (happened in December). Second, there are several specialties that do an "early match" in January (Neurology, Neurosurgery, Urology, ENT, and Opthalmology). This is how someone could have already matched. Match day for people who are going into other fields (and are not in the military) is on Thursday, March 21.
Hope this info helps...