I believe this affects network-manager more than glibc, as network-manager itself should deal with MTU discovery and setting the right value for the network.
That said, NetworkManager already has some amounts of logic to take care of that.
I'm also marking it as Confirmed since so many are seeing this issue, and seeing how there is a workaround that seems to take care of the problem for pretty much everybody.
To complete this bug, could you please provide the contents of /var/lib/dhcp3/dhclient.eth0.leases or /var/lib/dhcp3/dhclient.leases, if any? The contents of /etc/network/interfaces would also be good to have. Thanks in advance.