NETMODE has "transparent", but I'd like to take that further. Alva "Skip" Duckwall did a presentation at DEFCON 19, explaining that a MitM simply needs to wait for an Ethernet frame from each side of the bridge. Then you note the two MAC addresses, and simply modify the NICs at each end to match the opposite side. By doing this, your MitM simply disappears from the network cable.
I don't have my Squirrel yet (but it's ordered), but I figured I'd throw this out to the world and see if anyone else would like to have this added as a NETMODE.