the ultimate example of this trend might be the Infinity seats...




As far as just sitting on the rails... I'd swear that I'd seen a show bike at NAHBS that had a saddle with two parallel rails for the rider to sit on (in theory). I think it might have been a Cherubim bike, but haven't found an image of it. Maybe someone else has seen it too?

Steve in Peoria
(a couple of friends swear by the Infinity saddles, mostly because of issues with saddle sores)