Currently we put portage into /usr/portage and all related stuff is to be in the subfolders there (distfiles, binpkg).
I've always myself override these defaults in make.conf to point for /var/portage/ (not /var/lib because I never bothered enough how to make world and config files to be put elsewhere :P). The only reason why we have this currently in usr is that bsd ports put their stuff in there and I suppose Daniel just did the same. With respect to reality how stuff is done in the linux land all the variable data should be in /var so we should adjust and move it in there too. What would you think? Cheers Tom