I don't expect another revision until 2015, 2014 the earliest. I remember the DS lite came out in 2006 and DSi didn't come until 2009 (NA/EU/AUS) if I remember correctly.
I don't really see what they could truly revise aside from 3D tech, cameras, and a second nub
I think it's clear that Nintendo is(or will start) doing the Apple/Samsung super successful yearly revision with the 3DS now.
Just like the Galaxy revisions Nintendo can upgrade many stuff every year,the shape, the battery life,maybe better slidesticks,better screens..ect revisions are good for Customers,games and for Nintendo.