Received a Kramer birthday card last June from my former boss.....
I recall a Howard Stern/Seinfeld conversation from last year where Jerry admitted (or Howard teased) the cards as to being a marketing concept thought of in the first/second season when the show wasn't fairing too well (by "network" standards, that is--not ours!). By the time the cards came out (last year, maybe before) they didn't need them for the publicity factor anymore!! Anyway, I enjoyed my card when I received it last June because it was the first "Seinfeld" merchandise I had ever received.
--tara
P.S. While I was in England last week, I saw a commercial on SKY Television (wireless cable/satellite system in Europe) for Seinfeld. The show runs every Saturday at 9PM on SKY TV.