BMI do this
I travelled from Edinburgh to London yesterday on BMI with one of these "boarding passes".
It's a bit bizarre, actually: at security, and at the gate, they put the barcode under their scanner to validate it. When you get on the plane itself, the steward(ess) wants to do a visual inspection of the barcode (as well as the text part of the message). What (s)he hopes to learn from that, I'm not sure.
As Adam says, above, though, it does seem a solution in search of a problem. Tying my boarding to a plastic card already in my wallet would be soooo much easier.