If you are using Snow Leopard, then I believe the OS version you report in your issue report should read 10.6 not 10.4.6.
Snow Leopard has had a number of issues with fonts and this is more of a problem with the OS rather than FileMaker so you may need to research this on an Apple support site to learn more.
My understanding from other posts is that certain fonts and/or font suitcases that worked just fine with older versions of the Mac OS, cause problems for Snow Leopard. Most posts to this forum on the subject indicated that finding and removing/replacing duplicate or damaged fonts/suitcases solves the problem.
Thanks so I have a direction to go. Actually I was afraid that it ahas to do with Snow Leopard (which is indeed 10.6.4) - I'll try to isolate all unwanted fonts using Mac's Font-Management-Utility (Schriftensammlung). Will come back to report my experiences