Allen Kitchen and Allan Bloom recently posted to the Info-Mac Digest, warning readers about a potential credit card scam that may affect many Macintosh users specifically. They report that a company called Elite Concepts is impersonating MacUser magazine and calling MacUser subscribers, informing them (incorrectly) that their subscriptions are about to expire and offering supposedly "special" renewal rates that are in fact higher than the rates listed in MacUser's pull-out cards.
Although I doubt Elite Concepts is legitimate, I was unable to reach anyone at MacUser in time today to confirm anything. However, even if this group is legit, they have two major strikes in incorrect expiration dates and higher than normal prices, and a third, in my opinion, of pushy telemarketing. There's no reason to deal with them, just wait for your many renewal notices from MacUser and take advantage of one of the deals there. If you do get suckered into giving these people your credit card number, call the issuing bank and have them cancel the charge - then confirm in writing for safety - that's one of the advantages of paying by plastic.