(March 2012): As mentioned in Mike Morearty's answer (which I upvoted), git 1.7.10+ will support this feature.
Original answer (October 2010):
Currently, no.
As I mentioned in Is it possible to include a file in your .gitconfig, you already have 3 separate gitconfig for you to get your settings organized:
$GIT_DIR/config
Repository specific configuration file. (The filename is of course relative to the repository root, not the working directory.)
~/.gitconfig
User-specific configuration file. Also called "global" configuration file.
$(prefix)/etc/gitconfig
System-wide configuration file
Config File inclusion was discussed in May 2010, and a first patch was written by Ævar Arnfjörð Bjarmason, but I don't see this patch in one of the latest "what's cooking in Git".
