Force default gateway for a specific connection?

[Thomas Bechtold <thomasbechtold jpberlin de>]

Hi,

is it possible to tell NM to use a specific connection always as default 
gateway?
in my case, i use a modem and a lan cable and i want to use always the 
modem as default gateway. the reason is, that sometimes a firewall 
blocks some LAN traffic so i always want to use the modem connection as 
default gw.

TIA

Tom