Oh, there's another (probably slightly unprofessional) trick here. I use this in two home setups for two cheap reasons.

1) I want to use a firewall appliance but don't want to buy another wireless access point. Putting the 'all in one' router in to bridge and setting up as you've pictured above puts it's wireless out of use

2) My Firewall is in my garage (actually on an ESXi host). In the past, I didn't think I was staying in the house for a long time so I only ran 1 length of CAT5 from the house in to the garage. I'm too lazy to expand the hole and run another length!

If the ADSL router/modem is in passthrough mode and the WAN port on the router/firewall is set to PPPoE login ONLY, you can connect it all in to the same switching fabric. Just pop the ADSL router/modem web interface on to your LAN subnet and away you go.