(10 May 2012)
The ethernet plugin just reads values off of ifconfig. At high speeds (ie more than 50Mb/s) these counters can roll in the five minute interval, rendering them useless. Use the ip_ plugin instead.
Add to /etc/munin/plugin-conf.d/munin-node (normally it is already done in the last versions):
Add iptables dummy rules to collect stats on a particular IP:
iptables -A INPUT -d <ip>;iptables -A OUTPUT -s <ip>
Install the plugin for this ip:
ln -s /usr/share/munin/plugins/ip_ /etc/munin/plugins/ip_<ip>