Somewhere this week glibc 2.4 will enter testing, together with gcc 4.1 and a matching binutils. Glibc 2.4 has no support for linuxthreads, which means kernel 2.4 is no longer supported.
This means, that if you use testing on a 2.4 box, you should not upgrade to packages in testing after the glibc 2.4 package has gone in.