Libc6 is upgraded on Debian Sid and other distro updates. This set of objects is the very heart of our linux. If it ails, well heart failure can be fatal. The new libc6 is fine. It requires newer 2.6 kernels but I assume we are all onto these kernels already so no problem here. Or is there? A while back, jackd audio users were recommended to place a LD_ASSUME_KERNEL=2.4.19 in their /etc/profile or environment setup. (I have seen this recommended for other packages as well) The bad news is that this is a heart-attack. If it be in the profile, it breaks bash and any application run through bash. Other stuff make work OK but one might not be able to log on if bash or sh (symlinks to bash) be the user shell. (Boot up that knoppix CD, mount rw and usermod -s csh username) If this is set deeper down, well, time for panic. I do not know if the LD_ASSUME_KERNEL is still relevant to jack. However, do get rid of it before upgrading.