As much as I sometimes think of the airlines as the "enemy", I'd probably have a hard time ethically keeping the miles. To me, this is almost like finding money when you *know* who it belongs to, but you know they won't come and ask YOU if you've seen it.

To me this is different from "working the system". Airlines use the "system" to jam us up, and we sometimes use it to jam them back. Both adversaries recognize that there is a game to be played regarding airfares, miles, status, upgrades, etc. - may the best man win. But I have a hard time using that adversarial relationship to justify taking something because of an error. Doing nothing and playing dumb doesn't make it any better.