That's correct. We've addressed this question a number of times on the boards.
Everything posted on the DISboards or in your PMs resides on a big database. Anyone with access to the database would be able to read those.
Those people with access to the database would be: 1 webmaster (Alex), and a group of people at our hosting site who have probably never heard of the DISboards or have any idea what we're posting about.
Moderators do not have access to PMs, and I don't have access to them. Alex could technically delve through them if he wanted to spend the time required to do that, but I can't imagine any scenario where he would want to.
There are tools that we could install to make it possible for certain admins to more easily read private messages, but we have not done this and do not see any reason to.
So the answer is that it is technically possible for a very limited group of people to access PMs, but I am not aware of any instances where this has happened.
The thing I'd point out here is that anything you're saving to someone else's server (your emails, instant messages and files at work, on any forums you participate in, through any email service you use like AOL, Hotmail, Yahoo, etc.) is information that is available to the people who maintain that system.