lammert - 10:42 pm on Apr 17, 2010 (gmt 0)
I switched servers just two weeks ago. I started with setting the TTL to 300 seconds about 2 days before the transition.
When I changed the IP address to the new server, almost all human visitors picked up the new location in terms of minutes. Only some search engine bots, most noticeably Baidu and Yandex had cached the old IP address and hammered the old server for several hours. With this transition I haven't seen significant caching by ISPs anymore, which is a big improvement compared to transitions I did some years ago where human traffic to the old IP address died only after several hours to days.