I have the same problem on my Squeeze desktop (samba 3.5.5~dfsg-1, libkrb5-3 1.8.3+dfsg~beta1-1). I'm able to update the DNS successfully on a lenny schroot (samba 3.2.5-4lenny13, libkrb 1.6.dfsg.4~beta1-5lenny4) with the same configuration on the same machine. Regards, Wagner