DNS is really a give / take kind of thing. You can switch your DNS at most webhosts and it will start propagating on the internet within 10 minutes, but that doesn't mean that it happens for all your surfers at exactly the same time.

You can push your DNS out in only a few minutes, but that is just you broadcasting that you have a new address on the internet... that doesn't mean that every DNS server in the world is listening or asking.

We recently switched all of our DNS and it was interesting watching the bandwidth switch from one host to the other. About 80% of our traffic switched within 10 minutes of our DNS update as our new host had a very low TTL setting. Obviously so do about 80% of webhosts. But there was still traffic at our old host for several days after the switchover. There were random ISP's around that just didn't query the DNS to find the new address for a while.

So even if you are broadcasting the new address, you have to wait for every ISP to update their DNS to. We even had one Charter Cable location in Ohio wait 24 hours to update.

Then we had a few other customers who actually had a program on their computer that cached DNS and they had to manually flush their DNS in order to find us at our new home.

So most registrars (like directnic, godaddy, moniker, etc.) all have very short time to live settings and your host probably does to... its the rest of the world that has to comply.