Definitely take the 100-400L. You can never have too much reach for Yellowstone. Even 400mm on a crop body will be too short at times, especially in Lamar Valley and Hayden Valley.
I would take both camera bodies, too. Leave the 100-400L on the 7D, and 17-70 OS on the 50D. You will want to have your telephoto mounted on a body as you drive around the park, as you will come across lots of wildlife. Having two bodies will save you lots of lens changes, as you can use the 17-70/50D combo for your landscape shots. If you take a single body, you'll be changing from your 100-400L to the 17-70 every time you want to stop for a landscape shot. Remember -- a mountain or waterfall will wait for you to change lenses, but a grizzly bear or wolf will not.
I have been to Yellowstone five times. Like Rodney, about the only time I ever use an UWA lens -- EFS 10-22mm in my case -- is in the geyser basins. Even then, I find 17mm on a crop wide enough for almost all of those shots, too.