Standard Bullitt, Kink Cielencki, DBI Patriot are your best bet, but if you are looking for something lighter than that go to http://thickbikes.com/ where they build you a custom frame out of any material that you would like.
Don't get aluminum unless you are a racer, aluminum is not made for anything other that racing, the new S&M L.A.F (light as fu*k) frame is if not the lightest, one of the lightest frames on the market, its realy nice and weights in a 5 lbs. alot of good features to, like integrated head tube and the chain stay tubes are welded lower on the bb shell to accomodate a chainstay mounted brake and still run a small gearing
It will be the most expensive 10 minutes you've ever spent on a bike. If the same OS tubing that's used on my tandem was used, the front triangle would be plenty strong. But I don't see how they can make the back end strong enough to take much abuse. Too bad, because I'd really like to get myself a custom Ti BMX frame.