In order to properly answer this question, it's important to understand how the Internet works. While this is a very simplified explanation, it will hopefully give you the idea.

Although you may be used to typing in a familiar, easy-to-remember name to get to your favorite website, in reality, each site has a unique IP (Internet Protocol) number assigned to it.

Basically, every company that provides any kind of internet service uses something called a nameserver. The function of the nameserver is to interpret those names and turn them into the appropriate numbers.

Each company who provides web sites (and there are a lot of them!) send regular updates about which domains are on their IP addresses--Web750, for instance, updates twice a day, at noon and at midnight Pacific Time. 

All of these updates are gathered together, consolidated, and the information is sent out to all Internet Service Providers, who then update their own servers with the list. Depending on the provider, this update can happen a couple of times a day, or once a day.

Thus, a change, or "IP Propagation" as it is more commonly called, can take 24-48 hours to reach every user on the Internet. If you have just set up your site, give it at least that much time before reporting trouble, especially if you have some users that are reaching it no problem.