I think ACPM is the most reliable source of information. But, I also know that there is a LOT of variety in the programs. Some are very focused on public health and adminstration, whereas some are very clinical. For example, some program will assume (formally require) that you have completed a full primary care residency (rather than the universal rule of requiring a PGY-1 year only).
One of the nice aspects of ACPM is that you can join their Resident Physician Section as a resident in any field (I'm in IM, not PM yet). I attended their national conference last Feb, and found that it was a good way to get more direct exposure to the field. The one aspect about the conference that I didn't like was that their were a number of concurrent subtracks meeting, so it was often difficult to choose what sessions to go to (since I'm not "differentiated" into a particular area, like occupational med, "lifestyle medicine", public sector, global health, informatics, industry, etc).
I'm on some committees for the RPS this year, and one goal is to have more outreach to medical school campuses (there's a regional liaison for each section of the country - if you want, I could PM you some contact information). But, I still think it's hard to get information to residents in other disciplines who have never been told that PM was a career option.