I thought I had read that it was not weather sealed, but was not sure. So I grabbed the first official Canon site from the search results thinking that would be authoritative enough. I guess not. I was a little skeptical when it was worded with the impression that all "L" lenses are weather sealed... which I know to be untrue.
If you are not in the rain or snow much, then probably not a problem for you. It would be an issue for me if I did not have an alternate sealed lens in the same range. Rotary seals work pretty well, but seals for linear motion tend not to work well such that debris and water can be dragged through them easier. So I would think TS-E's would be particularly pour at keeping water out with 100% certainty. But if you do not get it wet, then it is not an issue.
Sorry for the misinformation.
Pat