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The last time I renumbered, I found that quite a few people were not honoring the TTLs I put in my DNS zone files. [...] Custom customer zone files hosted elsewhere?

Do not forget that applications have their own caches, too, and they typically ignore completely the DNS TTL. A typical Web brower calls getaddrinfo() once and use the IP address as long as it is not restarted. Not to mention java's caching which has screwed me up more times than I care to think about. I sincerley wish Sun had disabled it by default- I really don't think it's the JRE's responsibility to cache name service lookups- at least not by default.

-Don