I think Cheney was mistaken. I don't think he intentionally lied.
Why? Because if Edwards remembered meeting Cheney previously then he was a fool for not saying "Mr. Cheney, perhaps your memory is failing you but we met just last year at that prayer breakfast as well as on X and Y."
But he didn't. And if Cheney had really met Edwards in a memorable way and knew it, then his line in a live debate would have been far too risky.