It appears there is a problem with FreeType and Mac fonts. By Mac fonts, I mean any that are packaged as Mac suitcases (TrueType and PostScript) or the new OS X dfont. In particular it seems to affect multi-font packages, so could affect OTF fonts if there is more than one font in the OTF file. The problem - segfault (crash).
I never noticed before because I only ever use flat, single-font fonts, like OTF and Windows/Unix style TT and PS fonts. And none of the tools I use have any mechanism for specifying the style or face anyways - it's expected that the style is a part of the font file name chosen, ie Helvetica-Bold.otf. But, FreeType is supposed to default to the first face in a package, whatever that may be, yet it doesn't work.
Until I can figure it out, you should use only flat, single-font fonts with GD, ImageMagick, MapServer, GRASS and QGIS. OTF fonts are mostly OK, that is when there is only one face in the file. Unix-style PostScript fonts have a .pfb or .pfa extension. Unix-style TrueType fonts have a .ttf extension. You can use a font conversion utility, such as FontLab's TransType, to convert fonts to Windows/Unix format.
Interestingly, the FreeType that Apple includes with X11 does not have suitcase fonts enabled, but does have dfonts enabled. I wonder if they had problems also? I recall having an issue like this before, but I couldn't find anything in my archives…
Sorry for the inconvenience.