dhcp stopped working

I have a Linux machine connected to a cable modem and the ISP uses
dynamic IP numbers. I originally set it up with the 2.4 stable Debian
distribution and everything was fine.
When I changed to the testing distribution dhcp stopped working and I've
tried loads of things but can't it it to set the network up on booting.
If I load the currently leased IP address as a static one and then fire
up dhclient manually it communicates with the DHCP server.
The dhcp.conf file is out of the box and has pretty much nothing in it.
/etc/network/interfaces contains
iface eth0 inet dhcp
so when I try and bring up the interface with ifup then chclient starts
and I get 'network is down' messages.

Any ideas?

Keith
email address contains an extra no-spammm.