>
> Someone I know who runs a moderately large website told me that some ISPs
> cache DNS for as long as a week without regard to TTL.
>

We track DNS disobeyers at Google, as we use it for load balancing (along
with many other large sites).

I'd be kind of surprised if any large/professional ISP disobeyed the TTL
that badly, because it would cause frequent problems reaching popular sites
like anything hosted on Google or Akamai. But randomizing the DNS request
isn't a bad idea.