Interesting. The real issue question may lurk here: Did the developer do it because its in his job description or because he felt strongly that he wanted to communicate with the client in that way? And how does that change things?
http://particletree.com/notebook/the-4-12-day-workweek/
Now, as the company grows, this may have to be adjusted. The cards did not start as some marketing stunt, and we never want that message to be sent to our users. When cards from developer X don't feel personal enough, we'll have to brainstorm some more creative ways to thank our users.