ip route add default nexthop via 220.127.116.11 dev adsl weight 1 nexthop via 18.104.22.168 dev bdsl weight 1
to Zyxel P-660R ADSL routers that have no idea I'm bonding them, and two of Julian Anastasov's patches from http://www.ssi.bg/~ja/, to wit 00_static_routes-2.6.39-15.diff and 01_alt_routes-3.0-12.diff, to ensure that when one hop's routes go stale because of upstream problems we switch to the other. (I can't rely on normal 'when the link goes dead' bonding-driver stuff because the link that would go dead comes out of the router. I could fix this by using bridging, but Zyxel's documentation for that setup is so appalling that I haven't tried to make that work yet.)
However, I am lucky in that my ISP (AAISP) provides direct support for bonding on the ISP end: any packets sent to my public IPv4 or IPv6 address ranges will end up being evenly scattered between my lines (which fortunately are of similar speed, see the weight above). So all I have to handle is outbound routing.