lo stesso problema che si è presentato a me ieri dopo l'instazzione da cdnetinstaller etch giornaliero.
Controlla il file /etc/network/interface molto probabilmente anche x te causa del problema.
Io come rutermodem uso uno zixel prestige ed il file x avere attiva la rete senza problemi deve presentarsi in questo modo:

# /etc/network/interfaces -- configuration file for ifup(8), ifdown(8)
#
# If you are using the MEPIS network configuration for kernel 2.6.15 and above
# DO NOT use this file to define wireless options
#

#mapping hotplug
#script echo

auto lo
iface lo inet loopback

# DO NOT EDIT BELOW THIS LINE
auto eth0
allow-hotplug eth0
iface eth0 inet dhcp
address 192.168.79.1
netmask 255.255.255.0
broadcast 192.168.79.255
gateway 192.168.79.1
#auto eth1
#allow-hotplug eth1
iface eth1 inet dhcp
address 192.168.79.1
netmask 255.255.255.0
broadcast 192.168.79.255
gateway 192.168.79.1
#auto eth2
#allow-hotplug eth2
iface eth2 inet dhcp
address 192.168.79.1
netmask 255.255.255.0
broadcast 192.168.79.255
gateway 192.168.79.1
#auto ath0
#allow-hotplug ath0
iface ath0 inet dhcp
address 192.168.79.1
netmask 255.255.255.0
broadcast 192.168.79.255
gateway 192.168.79.1
#auto wlan0
#allow-hotplug wlan0
iface wlan0 inet dhcp
address 192.168.79.1
netmask 255.255.255.0
broadcast 192.168.79.255
gateway 192.168.79.1
# DO NOT EDIT ABOVE THIS LINE

iface dsl-provider inet ppp
provider dsl-provider


con permessi chmodlevel 755

Come noterai ho risolto il problema importandolo dall' installzione simplymepis , senza tanti complimenti