Check out this impression of Jon Gruden by Frank Caliendo, on ESPN's Sunday Countdown. Even though his ESPN hiring was met with groans by NFL fans, he nails it with his Gruden-ish mannerisms and phrases. If you just listen without watching, one would be hard pressed to notice any difference in the voices at all.
The best part about this is that I could totally see Gruden doing a Pee Wee QB Camp, and the result probably isn't too far off from what it would be like. Gruden would be great with Pee Wee football players what with his endless supply of positive affirmation. But in all seriousness, this Gruden impression is pretty accurate… and pretty disturbing.