I had the 3600, but it developed issues, and AT&T replaced it with a 3801HGV. The firmware menus look a little different, but the options and information all seem to be laid out the same. Not sure about it not being able to bridge the 3600, though. Maybe they could replace it, if that turns out to be the case?
I had looked into this for some time, too, and finally stumbled onto this site, which was quite helpful to me: http://www.sbbala.com/uverse/index.html
It describes two modes for the AT&T Residential Gateway: bridged, and modem. I went the modem route, as I wanted my D-Link DIR-890L to handle all aspects of my network (firewall, IP assignments, etc.), so the AT&T box truly acts as a modem only.
Hope this helps!