To be honest, I bought the 24-105L a few months ago when it was on sale, but have not use it much. I will definitely give it a good test before the trip. Would like to do some video as well, so the IS might come in handy.
I've been contemplating whether to take the 17-40L along or not. From some of the suggestions here, maybe I should. Especially for Rome and Venice. For those places, would a 17-40L do the job, or should I look into upgrading it to a 16-35L II. I've been especially impressed with the 17-40L, that I don't know if upgrading would make much of a difference. Tilt shift has been suggested, but price might be out of reach.
Thanks again for all of the suggestions.