Just a small update regarding this issue: I do not want to invent anything new 
before we've the redesign of live-initramfs available (which is planned for 
september, see upstream's mail: http://lists.debian.org/debian-live/2009/07/
msg00012.html).

Anyway, right now we *should* have the following boot option available already, 
according to the live-initramfs manpage:

       ip=[CLIENT_IP]:[SERVER_IP]:[GATEWAY_IP]:[NETMASK]:[HOSTNAME]:[DEVICE]:
[AUTOCONF] [,[CLIENT_IP]:[SERVER_IP]:[GATEWAY_IP]:[NETMASK]:[HOSTNAME]:[DEVICE]:
[AUTOCONF]]*
           Let you specify the name(s) and the options of the interface(s) that 
should be configured at boot time. Do not specify this if you want to use dhcp 
(default). It will be changed in a future release to mimick official kernel 
boot param specification (e.g. 
ip=10.0.0.1::10.0.0.254:255.255.255.0::eth0,:::::eth1:dhcp).

Let's figure out whether this works already or if it doesn't work let's find  
the cause for the problem.