I'm a brand new user trying to start PMS for the first time on Snow Lion with the newest Java patch and newest PMS download (as of 21-Apr-2012), and have the same problem. I found a very similar discussion with the same error message here: http://forum.serviio.org/viewtopic.php? ... 6&start=10
It seems to be caused by conflicting versions of the slf4j api files. So PMS uses a newer one, the System has an older one, and it picks up the System version for some reason. A quick fix is to open up a terminal window and:
mv slf4j* t
Basically just moving the two slf4j files into a subdirectory so they don't get picked up. This might mess with other Java programs you run that expect those API files to be hanging about... but it hasn't caused any problems for me. PMS started straight up after this.