I'm not sure what options you get with Downtubes, but as long as you're thinking about internal hubs, my Dahon has a 3-speed SRAM DualDrive, in addition to a conventional rear derailleur, giving it a whopping 27 speeds.

This is my first year with this bike, and like any gear shifting system, it will need some re-adjusting after an initial break-in period. I've recently noticed it hasn't been shifting as smoothly as when it was new, so I need to either learn to adjust it or take it to the LBS one of these days.

When it's tuned properly, it works beautifully. Don't have to worry about dropping my chain, and there's no possibility of crossing the chain. In that regard, it's kind of forgiving, compared to a conventional derailleur.

You might want to PM uforgot if you want specifics on the Sturmey Archer, in case she hasn't checked back on this thread recently. Would love to see a posted response, though, since I love learning about other people's bikes.