Well keep in mind folks that everything you post goes in the mysql database (well except photos and such which the text still does). Being a non admin there is no knowing if some of it is encrypted or not, on my site when people register the password is encrypted which means as the site owner even in the database I cannot see the info. But I have a friend who has a blog which I signed up for his site and sure enough he got my password!
Fortunately he is trust worthy, but made me a little more careful about passwords and such. My law teacher always tells me "The Internet is Forever! Anything posted, even private can be pulled up by site owners, government, ect". So it would not surprise me one bit if admins/site owners can read my pm's. Does that bother me? No, because I have faith that the admins won't try anything crazy.
Plus I usually don't post anything that they can use against me. Now keep in mind, it is possible that Matt might have access to statistics rather than actual access because I know as a website owner I can see the number of messages for the encrypted stored data (the data that I cannot read). So you can't necessarily base it on the factor of the admins/owners knowing how many messages are being sent.
Also moderators/ administrators do not have access to the mysql, only Josh and Ryle (perhaps Matt) has access to it. I know this because of personal experience and reading some former sp posts.