Thanks Everyone. I think the bottom line question is has anyone else experience significant (photo ruining) blurring with the IS On while mounted on a tripod? I suspect the answer is no. It could be that this has been the case all along and I've been much better than I recall at turning IS Off when on the tripod. But I doubt it.

@Arnt---I was using the 2 second time delay on Saturday and in my subsequent test shots. This morning I listened and I could hear the IS whirling during those two seconds, so it should have had enough time to settle in.

Everyone else, thanks for you input. I will be contacting Canon and probably sending it in as I suspect there is a problem. I updated to the recent firmware, cleaned the contacts (both lens and body are ~1 year old) and ran a series of optical tests. Fortunately, it checks out optically. My AFMA are essentially the same (0 adjustment at 105 mm from previous and -1 at 24 mm from previous), aperture sharpness test looks great, and the AF consistency test was ~98%. I also tested the IS and it seems to be giving me the ~2-2.5 stops of IS it typically had (I have never been too pleased with the IS on the 24-105. IS is much better on my other lenses, but it is an older generation of IS). I also took a bunch of random shots around the house and they all turned out as expected.

But right after my FoCal tests this morning, I tested the tripod detection system again and instantly saw the problem. First shot with IS Off was sharp. The next four shots with ISO On while mounted on the tripod ranged from minor, but visible, impact to image ruining effect. I'll try to post the pics soon.

Thanks again...seems like it is time to send the 24-105 home for a bit.