Once upon a time Citi was the largest issuer of VISA network cards, but a dispute over card branding caused them to change their emphasis to MasterCard.
Correct, I remember the tag line, "Not just VISA, Citibank VISA"
My point is that I have a Citi Prestige & Citi AA Executive WE both MC and considered to be Citi's flagship credit cards and if they are not excepted at Costco then Citi risks alienating some of their best customers. It will be interesting if Citi will offer these two products in VISA flagged products as well due to the Costco deal.