I consistently find myself moving from place to place when working. Sometimes at home, Starbucks, or any other WiFi hotspot. And it's very important to realize that different ISPs may provide different DNS traces, which may not be necessarily valid across all of them. If ever you run across this issue, try issuing the following command :

dscacheutil -flushcache

This is the command for Leopard.

For Mac OS X Tiger, this is the command :

lookupd -flushcache

Hope this helps you too! It has really helped me especially during those mission critical svn commits I have to do! :D