I own two 70-200 f/2.8 lenses, one each with and without IS. If I were starting all over and had to buy a new 70-200 right now, I would think long and hard about buying the f/4 IS version. Not only is it lighter and cheaper, but it balances better on the 40D body. In my experience, the f/2.8 lens is so much larger and heavier than my 40D, it's just not comfortable. (I've tried it both with and without the battery grip.)
But if you have to have the f/2.8 lens, don't worry about the lens mount. It should be fine. Just make sure you turn the camera around backwards before putting the strap over your shoulder. It'll hang properly this way, without the lens sticking out and banging into things.