Is it possible that the /sbin/sysupgrade in 1.3.11 no longer understands .bin files?
I tried to 'upgrade' from 1.3.11 down to 1.2.5 via the web interface, which just sat and spun on the busy screen. Logging in via ssh, the box didn't seem to be doing anything, and after reading do_update.sh to see what it actually does, I tried running /sbin/sysupgrade by hand.
It complained that the wrt54gs .bin file in /tmp/up/upgrade was not the right kind of file (sorry, the exact message is no longer in any scrollback buffer here). I then uploaded the 1.2.5 .trx file to the box, and /sbin/sysupgrade was perfectly happy with that. (Yes, it's a wrt54gs router --- I used the same-labeled .bin file as for the original flash to 1.3.11 from the Linksys proprietary firmware.)
A secondary (primary?) bug here, also: the web interface apparently doesn't handle a failed sysupgrade invocation gracefully (since it just sits and spins).