Abe Fettig finally has a solution that allows using your OS X fonts in The Gimp. Well, almost, it should be ~/Library/Fonts rather than ~/System/Fonts:
In a Terminal window, enter
cd ~/.fonts; mkdir sys; mkdir user;
ln -s /Library/Fonts ~/.fonts/sys
ln -s ~/Library/Fonts ~/.fonts/user
which will create symbolic links between /Library/Fonts and ~/Library/Fonts (where your OS X TrueType fonts are installed) and the new directories in ~/.fonts (where GTK2 looks).
Then run The Gimp, and all your fonts should be available for the Text tool.