Once the nameservers have been updated at the domain registrar it takes up to 72 hours for this information to filter through the internet. What happens is that your domain registrar submits this information to the master record database and it is available there very soon. However, each ISP (the way you access the internet) does not use the master record every time they need to find a domain name.

What the ISPs do is copy the master record and then have their own database of nameserver information. When someone looks for a domain name they are given the information in their ISPs database which is not necessarily the up to date master record. The real problem is due to the fact that each ISP updates their database with the new information on the master server whenever they want to. Some do it every night, some every other day and some up to 3 days.  I have even seen instances where it could take longer if the ISP in question is having a problem with their DNS server syncing with a master server (very rare, but it can happen and often goes unreported for a long time)

So even though you made the nameserver changes it *depends* entirely on when your ISP updates the info in their database, which can take up to three days even though your account is activated right away. This does not mean you cannot access your account and transfer files--you can do that right away as well as set up all of your mail accounts and more, but until everyone updates to the new records some people might not find your site. The process of this filtering of the master record through the internet is called DNS propagation.