Thanks for the replies.

JS - I hadn't thought about panning. Thanks

Michael - I'm not, at the moment, in the market for a lens that long I was just curious why a lens that you would have to use a tripod for (speaking of the longer Canon L lenses at 11 + lbs.) would need IS.

Paul - I was unaware that you could use IS on the L lenses while on a tripod. Thanks for the 411.