I swear this question comes up every 6th months or so.
Rich has the right answer. From a technical point of view anyone with database access can read anything just like any tech person at your work can read your emails and AOL or Gmail staff can read your emails if you use their service and they really want to.
In order to do it you have to have server access. No moderators or webmasters have that server access as they have no need. The only people with server access are Pete, myself and some behind the scenes tech people that you have never heard of and have never heard of you. We all have way better things to do than wade through the 1000's of PM's that are sent everyday.
The honest to god truth is ANYTHING you write or post on the internet can be read by someone. Even the credit card numbers you put in on a "secure" site can be read by the technical staff somewhere.