If you are on SQL Server 2000 and you've been looking at your six CPU setup thinking "I'd hate to pay for the licenses to upgrade that" it may not be as bad as you think.
With SQL Server 2005 we introduced some license changes (note that there are no license changes from 2005 to 2008). One of the key changes for highly redundant solutions was that where the server has no active users against it because it is a passive server (i.e. used for DR or in a mirror), there is no licensing requirement.
Lets test this with a scenario (assuming CPU based licensing - it works similarly with Server/CAL licensing). Imagine you have a dual CPU server with a mirrored server (again with two CPU's). On top of this, you have a DR Server, also with two CPU's. In SQL Server 2000, you would have purchased SQL Server Enterprise Edition for six CPU's. With SQL Server 2005/2008, you wouldn't pay for the mirrored server (which simply replicates data unless the primary server is failed over to it).
Why is this the case? Because the licensing for SQL Server Workgroup, Standard and Enterprise allows you to install the equivalent number of instances as you license as cold or warm servers - meaning servers that are not accessed by users except when they are failed over to.
So what about the DR server? In the case above, it would need to be licensed. However, a work around for this would be to set up the mirror as a single CPU machine and the DR server as a single CPU machine and then license SQL Server for both CPU's on the primary server.
When would the passive server need to have a SQL server license? If a passive server is being used for anything other than simply absorbing data and information held in another system it needs to be licensed. An example of this would be when users are accessing the passive server for reporting or other sort of user access.
So if you are looking at upgrading to SQL Server 2008 from SQL Server 2000, and you have a highly redundant configuration - your licensing costs won't be the same as they were when you purchased SQL Server 2000.
Don't forget that we're just talking about SQL Server here. You'll still need to license Windows Server 2008 and any other products on the DR server.
Before you go and put together all your licensing around this concept, you'll want to take the time to read the product use rights - particularly the exceptions for SQL Server.
Note: I am not responsible for your licensing and can't legally advise you :-).