It took me a while to figure out how to set my SpeedStream 4200 into bridge-only/passthrough mode, so I thought I'd post it here in case I ever need it again, or, someone else googles it.
To program bridge only mode log in to the SpeedStream 4200 at http://192.168.254.254/ . User and password defaults are admin and admin. Click Setup then click Wan interface, double click the top item (mine is 0/35) then click next at the bottom of the page twice and you should see Bridge only mode at the top of the page, click bridge only then finish. "DO NOT REBOOT" now click DHCP for the right side menu and at the top of the page DHCP Server click the Disable button now reboot and you will have a bridge only modem.
--from allteltec in this thread.
This allows me to use my linksys router like I did before I got the SpeedStream 4200.
A faster way to get to the Bridge mode option is to go directly to http://192.168.254.254/brgmode.htm and click Apply. I do not know whether this will affect the DHCP setting, or if one even needs to bother with DHCP when using bridge mode.