Paul Vixie published patches for BSD/OS 2.1 which allow for a default route per interface and which remember the interface via which a tcp connection
was initiated. See ftp://ftp.vix.com/pub/vixie/ifdefault/ for more.

The patches to the userland programs ifconfig and netstat applied with minor 
work and the kernel changes to if.c and related headers were no problem. I 
did not manage to get get the patch for tcp_input.c applied because Jason 
Thorpe's excellent work on SYN caching and my lack of understanding of 
TCP kernel matters. 

Has anyone else looked at this? I would like to see this in the tree, the 
changes are small (the diffs for kernel and userland programs are less than 
450 lines) and the number of machines, like mine, which are becoming 
dual-homed because of cheap and fast cable modem based connections is growing 
quickly.

-- 
Andreas Wrede              Planix, Inc.
andreas@planix.com         Networking, System Administration, Consulting
http://www.planix.com      Toronto, Ontario, Canada