Font question - What is a .dfont font file?

From

http://docs.info.apple.com/article.html?artnum=25251

Data Fork Suitcase Format

One of the innovations offered by Mac OS X is that font suitcases can be completely stored in a file's data fork. All of the resource fork's data is stored in the data fork, which allows more efficient access to font data as well as the ability to copy font suitcases to and from file systems that do not recognize resource forks.

The data fork suitcase format is different from the data fork TrueType font format used by Microsoft Windows. Data fork suitcases contain all of the resources associated with a Macintosh font, including 'FOND' and 'NFNT' resources, which are used with QuickDraw Text.

Data fork suitcases must have the filename extension ".dfont." They may optionally be given the file type "dfon."

You should always keep the ones located in system/library/fonts I think, if you want to avoid future problems. A list can be found here:

http://docs.info.apple.com/article.html?artnum=301332