I don't know if this counts as an idiolect thing, but sometimes a really small community can develop speech patterns that show identity with that community-a "microlect", if you will. I'm sure we could all find this in action within our immediate families and groups of friends, but where I first noticed it in "linguistics" terms was when I was living in a student co-op in Berkeley. The house members were all really close and spent a lot of time together, and we definitely had developed a distinct code--some word-usages that were different from "outsiders", some codes of vocal pitch/intonation that could convey a lot. I guess a lot of this stuff was just "in-jokes" and references that had become so ingrained that just single words or vocal cues could invoke the entire reference.

I think situtations like this could actually have a lot to teach us about how language change happens on a large scale.