I have a similar dent (about 2 inches lower on the cross tube) in my Cannondale road bike due to a fall of the workstand. I agree with the others. It is not a big deal as long as the dent is "smooth" with no creases.

The Tubing on my wife's T 800 seems thicker and larger diameter than my road bike. If this is the case I definitely would not worry about it.