The old Hardware Compatibility List which was renamed the "Windows Catalog" is no more. When creating a Clustered Solution, you need to have a system that is Certified for clustering or there will be no official support. This has been a point of pain in the past as organizations purchased a clustered solution and then one of the nodes has issues after a year or two. Getting an exact replica of that node is often impossible and impractical. Try explaining to the big wigs that you need to purchase a new system that is 2 years old - not very compelling.
In Windows 2008 and SQL 2008, there is no need for a certified solution. There is a program called clusprep.exe that you run on the nodes of the proposed cluster to test/prepare the nodes. If your system passes, it can run on a cluster.
One drawback of this is that when failing over to a node of lesser "girth", the user experience may suffer. Also, with this change, the base of clustering is changing which means that your upgrade to the 2008 flavor will take some more planning.
If you attended the PASS Community Summit in Denver, you can view the streaming session online on Clustering. It was very educational.