I've talked to a few people who are looking to upgrade from SQL Server 2000 recently, and it seems that most would rather go onto SQL Server 2005 than take the complete step up to SQL Server 2008. Most people also plan on changing from Enterprise Edition in SQL Server 2000 to Standard Edition during the upgrade. I can't say that I really fault people for trying to save money, but my biggest recommendation to everyone I talk to that is thinking like this is to contact Microsoft Licensing directly and get some more information before making the License purchase, and here is why.
If you read the Licensing FAQ for SQL Server 2008 there is an interesting question at the bottom of the list:
Q. How do I downgrade to SQL Server 2005?
A. If you require SQL Server 2005 for existing solutions, you can still acquire additional licenses by taking advantage of the SQL Server 2008 "downgrade rights." The downgrade rights enable you to purchase server plus device CALs, server plus user CALs, or processor licenses for SQL Server 2008 and install and use the previous version for a reasonable period of time.
This is a nice little nugget of information, especially for people who are still just waiting for Service Pack 1 for SQL Server 2008, which seems to be the industry expectation. So what exactly is a reasonable period of time? I can't tell you that, it would take all the reason for you actually contacting Microsoft Licensing yourself, but I will provide the information you need to find out. All of the information to contact Microsoft Licensing is available online on the SQL Server 2008 Licensing Page.
Find answers to all of your questions about licensing SQL Server 2008.
In the US, call (800) 426-9400 to speak to a Microsoft representative.
In Canada, call (877) 568-2495.
Before making a licensing change, I'd recommend contacting Microsoft to get further information, but I would also recommend reviewing the features comparison in SQL Server 2008 for Standard and Enterprise Editions. A lot of the really nice new features in SQL Server 2008 are Enterprise Edition Features, not to mention that Partitioning is Enterprise Edition in SQL Server 2005 and SQL Server 2008.
SQL Server 2008 Detailed Feature Comparison Standard vs Enterprise