I think a few schools have had orientation starting July 31 - and I think some Eastern schools don't start until September. I think my experience this year was pretty typical for a state school - orientation started Wednesday, August 10, and classes started the following Monday, August 15. I don't know about other medical schools, but we have 18-week semesters while the rest of the University has 16-week semesters.
The University of Oklahoma College of Medicine - Class of 2010
We started August 14th, and my other friends in med school started around the same time. One friend did start at the very end of July, but that's not typical.
There are a few schools, such as Mayo which start at the beginning of July and some that start pretty late in September (trimesters), such as the University of Chicago. But like the others said, most are in the middle of August to the beginning of September from what I have gathered.