During the integration process with NWA I remember seeing many FAs wearing "AFA" (i.e. I would like a union) pins, and before that the "Keep Delta My Delta" pins. So, it doesn't seem all that unusual for employees to add one pin to their uniform to express a preference. (I don't remember ever seeing more than one at a time, however.)
I late August and in September of this year (2013) I remember seeing seeing several FA's with pins from their favorite SEC school/football team. It resulted in a slightly modified beverage service. For example:
FA: What can I get you?
passenger: A ginger ale and a Roll Tide.
or
FA: What can I get you?
passenger: A Coke and a War Eagle.
etc.
These were all flights into or out of ATL, so perhaps they were mostly ATL based crews and thus the preference for the South Eastern Conference (SEC).
I have also seen pins with a military theme (e.g. a USAF logo pin), holiday pins (a Christmas tree), etc.
I doubt I would have been able to figure out the flying D pin meant something like "I'm happy without a union" without this tread, however, so thanks for the raw info.