Interesting blurb on cardtrak.com:

All the major credit card brands are aiming for the top of the heap with super reward cards for the super wealthy. However, a new study has found that the American Express' $2500 per year "Centurion" card reigns as the most prestigious credit card among the super affluent. New York-based Luxury Institute says MasterCard's "World Elite" ranked third while VISA's "Signature" came in sixth. The study found that younger consumers are more likely to choose "American Express Centurion" or "MasterCard World Elite," while older consumers are more likely to choose "VISA Signature." The "2007 Luxury Brand Status Index" ranks consumers with a minimum net worth of $5 million and $200,000 in annual income. The national sample of over 500 ultra-wealthy American consumers, with an average income of $775K, and average net worth of $14.4 million, was surveyed online.