It is Suitcase doing this, and as yet i haven't found a way around it- I have yet to trash font book yet (i never use it though) if you close the offending app, and re-open it, the problem goes away. It is caused by suitcase requesting to open a font that is owned by the system, but it is ok if the font is already open and in use (and then you open an app that requests that font). I presume that if you replaced your system fonts with postscript ones you want suitcase to have, the problem would be solved- or wait until extensis fix the issue. Dont worry though, it seems a few people have this problem as well.
Cheers,
Chris