The bridge-utils package provides hooks so you can set up a bridge interface in
/etc/network/interfaces. Here is an example:

auto br0
iface br0 inet static
	address 192.168.1.2/24
	gateway 192.168.1.1
	bridge_ports eth0 eth1

Note that you do not have to specify separate iface stanzas for eth0 and eth1.
For more information, type "man bridge-utils-interfaces".