Network interface eth0 not up at start on Debian 6

add following line to /etc/network/interfaces :

allow-hotplug eth0

Source:man interfaces

lines beginning with "allow-" are used to identify interfaces that should be brought up automatically by various subsytems. This may be done using a command such as "ifup --allow=hotplug eth0 eth1", which will only bring up eth0 or eth1 if it is listed in an "allow-hotplug" line. Note that "allow-auto" and "auto" are synonyms.