wget http://distfiles.gentoo.org/releases/snapshots/current/portage-latest.tar.bz2 downloads portage-20111126.tar.bz2 ; tar xvjf portage-latest.tar.bz2 -C /mnt/gentoo/usr results in tar: Unexpected EOF in archive tar: Unexpected EOF in archive tar: Error is not recoverable: exiting now portage-20111126.tar.bz2 does not have a problem.
Looking at the directory listing in http://distfiles.gentoo.org/snapshots/, it seems that the file size for the 20111126 is about 5M smaller than the 20111125 and earlier snapshots: portage-20111123.tar.bz2 23-Nov-2011 19:45 41M portage-20111124.tar.bz2 25-Nov-2011 17:50 41M portage-20111125.tar.bz2 25-Nov-2011 20:00 41M portage-20111126.tar.bz2 26-Nov-2011 19:45 34M
tar failed weirdly. I've added some extra checking for that now, and fired a run to regen the files for 20111126.
the snapshots+deltas are regened.