Right now on our fonts we say they're LGPL. (Look for the LangName attribute in the .sfd file.)
We might want to make a modification to that. Here's what the FSF has to say about it: http://www.fsf.org/licensing/licenses/gpl-faq.html#FontException
Or, to just copy and paste that info: -----------------------8<----------------- "How does the GPL apply to fonts?"
Font licensing is a complex issue which needs serious consideration. The following license exception is experimental but approved for general use. We welcome suggestions on this subject—please see this this explanatory essay and write to licensing@gnu.org.
To use this exception, add this text to the license notice of each file in the package (to the extent possible), at the end of the text that says the file is distributed under the GNU GPL:
As a special exception, if you create a document which uses this font, and embed this font or unaltered portions of this font into the document, this font does not by itself cause the resulting document to be covered by the GNU General Public License. This exception does not however invalidate any other reasons why the document might be covered by the GNU General Public License. If you modify this font, you may extend this exception to your version of the font, but you are not obligated to do so. If you do not wish to do so, delete this exception statement from your version.
-Brian