I went to purchase a CO ticket earlier today with my MasterCard and the website asked me for my SecureCode, which (according to the screen) I would have set up when my card was issued. I didn't recall setting up that number, so I tried the three digit number on the back of the card. No dice and eventually I had to use my AMEX card.

Puzzled, I called my issuing bank, USAA, about this and was told that I did need to register and obtain a SecureCode, which I then did (so the message was wrong about already having a number). It seems that MasterCard started rolling out this program about 8-9 months ago and Visa is doing the same. It is up to the merchant about participation and it looks like CO is now doing so. So before you purchase a ticket on CO.com with your MasterCard, you might want to ensure that you have a SecureCode.