So I'm about to plunk down some money on a new Trek Soho (I don't mind being a guinea pig for the belt-drive) when the LBS starts going over the specs and I notice it uses a Nexus IGH instead of the Alfine. I assumed they carried over the same hub from the 2008 Soho 4.0, but I guess not. The LBS guy said the two hubs were the same internally, with the only difference being that the Alfine could accept disk brakes. I knew that the Alfine could accept disk brakes and the Nexus could not, but I had not heard that they were the same internally. After doing some research I have come up with a broad range of views. Everything from "They are identical except for the brake difference." To "The Alfine is lighter," "Tougher," "Smoother to shift," "Different internally," and "Holds gears better." The information at http://www.sheldonbrown.com/harris/shimano-alfine.html seems to indicate the Alfine is an "improvement" over the Nexus.
I have an email in to Trek, but I'm kind of expecting a "salesman" type answer as opposed to some hard facts.
Does anyone have a definitive answer?