While mulling over a confusing part of something I'm writing, I decided to browse the info pages of GPC. In the GNU section, I read this:
When you modify a free program released under the GNU General Public License, e.g. the GNU Pascal compiler itself, your modified will become Free Software, too.
I'm no lawyer, but I've read enough of RMS's articles regarding the GPL, as well as a few technical (but not necessarily legalese) discussions of the same, to understand that, when modifications to Free Software DO NOT need to become free themselves.
As I understand it, modifications to Free Software do not need to be released; but when it is, it HAS to be released under the GPL, if the program modified was released under the GPL.
[CC'd to RMS, in case I goofed, and therefore should be banished from society and be forced to use nothing but proprietary software.]