I just had another interesting problem. Logging into one server was very very slow... at first I thought it was reverse DNS, but no.
Turns out that ssh was trying to syslog, and I restarted syslog so it'd bind to ipv6 and v4 and suddenly things worked again as normal. This server had not been rebooted when ipv4 was enabled, it'd previously been blacklisted in /etc/modules.mumble, this is an old debian lenny box), so I'd modprobe'd ipv6 and thus none of the running system services knew about ipv6!
