Juniper SSG20 - Bridge ADSL to ethernet interface
I played with it a bit and it looks plausible. The starting point would be switching adsl0/1 into Layer2 mode. You can accomplish this by assigning this interface to one of the V1-* zones, let's assume V1-Untrust. Then you put one or more of the ethernet interfaces in the same zone and it should make a fancy DSL modem out of you SSG-20 :-)
I haven't tested this configuration myself yet since my ADSL card is being replaced. I will able to try this in a couple of days though and will post an update.