Again, bad host name. prep.ai.mit.edu no longer exists. What is more, it appears that the GNU sites have a problem with the file grep_2.5.1.tar.gz. It is MISSING. I was able to find a copy at http://ftp.club.cc.cmu.edu/pub/gnu/grep/... This is all still trying to get emerge -u system to work on a fresh install from the recent instructions and cd images.
grr, the GNU mirrors are still not back up. Pointing the ebuild to the site you mentioned and the gentoo mirrors