This might be an interesting idea, especially if the machine prints out a mark on the boarding pass in a predictable manner and location.

Photoshop would do wonders for those of us with home-printed tickets. And we all know that terrorists can't buy Photoshop to modify their boarding passes...

We're already jumping through too many hoops and spending too much money to solve a problem that hasn't manifested itself and is extremely unlike to do so in the foreseeable future. We don't need to spend even more money on this.