I finished some shots tonight to test what I thought was the IS being unstable, thinking it may have been due to operating well below it's intended temp range (-20C tonight, -10 the first time I suspected it). There is a clear difference in the full frame printed at 8x10. Looking at details, the IS was drifting vertically.
There was a comment earlier in this discussion that the IS in some lenses (or Canon in particular was being referred to) hunts when on a tripod. Is there more information on this somewhere? It didn't pop up in a google search.
The lens I used as the Canon EFS 17-85 IS.
It's interesting, but IS may require a certain minimum of shake to avoid creating movement on it's own!