well, atleast don't do *anything* that would make you go back a version of glibc. i guess this is a bug (not sure if it's gentoo specific or not), and your entire system dies when you go backwards a version in glibc.
in order to prevent this, try emerge -pv glibc and see if it has D in the left hand field. if it does, don't emerge system.
anyway, it's just my warning... after working so long and hard to get my gentoo stage1 install next to perfect, this screwed it up =\.
afaik there is no known fix for this problem other than reformatting the filesystem. even reinstalling the bootstrap was useless to someone.
here's a thread for more information:
note: this may apply to other distributions as well, but i'm not sure