Wired Network - device not managed

Once I manually made /etc/network/interfaces like this
auto lo eth0
iface lo inet loopback
iface eth0 inet static
address 172.20.24.222
netmask 255.255.255.0
network 172.20.24.0
broadcast 172.20.24.255
gateway 172.20.24.254

/etc/resolv.conf
nameserver 202.22.11.39

/etc/init.d/networking restart

to specific a static ip address.

When I wanted to configue it back to DHCP,
I could not use NetworkManager Applet to manage the eth0.
It showed "Wired Network -> device not managed".

Note:
For this, I learned that to configure,
just use the NetworkManager Applet GUI
( /usr/bin/nm-connection-editor ).


Again later I connected Ubuntu to a ADSL modem using pppoeconf
( https://help.ubuntu.com/community/ADSLPPPoE )
and later connected back to a dhcp hub,
the same error appeared.

The solution
========
that work perfectly on both is
changing the content of /etc/network/interfaces to be:

auto lo eth0
iface lo inet loopback

OS: Ubuntu 9.04