I have two revisions of glibc installed. The older one is glibc-2.2.5 and was merged awhile ago with portage 1.8.8. The second, glibc-2.2.5-r1 is the more recent merge and was merged with portage 1.8.9_pre* (judging from the date and the existance of a SLOT file) If I try to unmerge the older glibc package (glibc-2.2.5), it tries to remove all the symlinks in the package from /lib. All other files are left intact but the symlinks are removed, leaving a broken system. I have not noticed this behavior while unmerging any other older package. I had to use 'sash' to put enough of the symlinks back so I could remerge glibc and have the symlinks all recreated.
fix is now on gentoo-src cvs. This was a long-standing bug in Portage and I appreciate this report greatly since it was desperately needed.