But isn't IS a feature as opposed to changing the aperature or focal length? I dunno, maybe I'm just having a bad day at work and my subconscious is looking for an argument to distract me even though I say I'm not...
IS and USM are both features, but they change the name of the lens, not the version. So we had a 70-200mm f/2.8L (1995) and a 70-200mm f/4L (1999). Then there was a 70-200mm f/2.8L IS (2001) then a 70-200mm f/4L IS (2006), and most recently, a 70-200mm f/2.8L IS II (2010).
Other examples are the super tele lenses, say the 400/2.8 lenses. The first was the 400mm f/2.8L (1991), followed by the 400mm f/2.8L II (1996). Then they added IS, but we didn't get a MkIII, we got a 400mm f/2.8L IS (1999). Now, this year, we have a 400mm f/2.8L IS II (ok, I'm using 'have' metaphorically, since at $11500 a pop, not many of us actually have one...).
Or, look at the 75-300mm f/4-5.6 consumer zooms. There are actually 7 versions - original, II and III, original USM, II USM, and III USM, and IS USM.
Bottom line, adding IS with no change in focal length or aperture still restarts the version numbering.