AFAIK, nothing changed in this regard between CS4 and CS5.5.
I'm not very comfortable with AppleScript, but what happens if you put quotes around "JPG"?
Ja, that's really strange!! I spent already a lot of time in research on this.
If I put quotes around ("JPG") I receive an error: Das gewünschte Exportformat wird von dem angegebenen Objekt nicht unterstützt. (The export format is not supported by this object.)
Does this make sence?