Admins do not have access to the PM database except under very rare circumstances such as a serious complaint being made, we will then make efforts to access the database code and sift through enormous amounts of 'data' in order to establish the facts (to this date I think this has only happened once.) Occasionaly if there is a technical fault we will access the database but only in order to delete any erroneous code which may be causing problems (weve done this a few times in order to get the forum going again) we do not however scan through anyones PM's as generally we know what we are looking for (ie the 'buggy' code which is causing the forum to trip)
So basically no, PM's are not routinely scanned/read or snooped upon in anyway except under the above circumstances, even then alot of the data is not in a human readable format, it isn't all encrypted but neither is it easy to read normally.
Hope this helps. Out of interest, may I enqire as to what those issues were on the other forum you vist? Generally speaking I mean, you don't need to divulge specifics. It's interesting for us to know about these things in order to avoid similar problems cropping up here.