[Scott MacVicar]

They're meant to obey TTL.

Personally I add entries to my
/etc/hosts on my linux box
and
c:/winnt/system32/drivers/etc/hosts

So it resolves instantly.

Another tip.
Update your 2nd nameserver first and wait 2-3 days and then update your first nameserver, no one will notice its down but it might cause confusion with non static content like forums.