Hmm. No one has suggested the Xtracycle (http://www.xtracycle.com/html/home.php). Not a rack, but if you truly need to haul big loads, it's a good way to go.

Also, check out A.N.T.'s racks (http://www.antbikemike.com/basketsandracks.html). And I've come across custom rack builders on the web (there's one in Texas: anyone?) that could probably make an extra long rack.

edit: A.N.T. says that their "Baskets and racks are built for ANT frames only." Not sure if that means they won't fit other bikes, but just fyi.