Ubuntu 7.10 networking problem

[Alex]

Hi,

Please help a noob to get his internet connection going in this Gutsy !!!

ok so i am in an internal network where i get ip address assigned from a 
dhcp server based on my mac.
in the previous release of ubuntu i used to edit /etc/dhcp3/dhclient.conf :
...
send dhcp-client-identifier 1:0:<my mac>  (1:0: or 10: ... not sure 
which one worked)
...

now in gutsy i additionally changed /etc/network/interfaces so that it 
contains the following lines:
...
auto eth0
iface eth0 inet dhcp
...

still ifup eth0 gives me no ip from the dhcp server.
More background:  <my mac> was set using ifconfig eth0 hw ether <my 
mac>  (becuase <my mac> on which i am granted an IP is not actually my 
nic's MAC)


Any suggestions on what i am doing wrong?

thanks,
alex