A couple of years ago I read (in UA produced materials) that 1K customers were exempted from showing the credit card upon check in. (This was before Mr. EZ).

The few random times I've been asked for a credit card (I always buy my work tix on my boss' CC) I have said "As a 1K I don't have to show the credit card" and that was the end of it.

Also, if you buy with a UA Mileage Plus Credit Card I don't think there is a prompt either. (The other good thing about this card, is you can go way over on your credit limit if you're buying UA tickets ;D)