http://lxr.mozilla.org/mozilla/source/config/fast-update.pl

??

If you don't need to do checkins, CVSup is your friend. Much faster as it can do multiplexing downloads.

?? "make -f client.mk checkout" will not re-checkout the directories it already has, even if the process got interrupted for whatever reason. of course it still has to check if a checkin happened to those files, but that should be reasonably fast... just run it again.