Latest Glibc from CVS has segmentation problems.

[Alfred M. Szmidt]

   What if I want features not available in the latest released
   version, such as NPTL?

Then don't install a untested version of libc in your root, use a
chroot, and check that it works before using it.  This is a normal
process for any kind of testing/usage of unstable (critical)
componets.  You just don't "make install" something as libc from CVS
without knowing that it actually works!