Oh, yeah, a single speed. I think it might be interesting. Problem for me with a bike with gears is that I'd be forever adjusting them. That's the reason I don't get a Gillette razor that's adjustable, I'd never be able to decide on the stetting; it takes me at least ten minutes just to set my watch...
I've always thought that good bikes (i.e. light etc) are really in a way counter productive for fitness; as they require less energy. However, I suppose there's a limit; I'm not about to get out on a penny farthing while wearing a diving belt, hob nailed boots and a one piece rubber suit...