This month's Redmond Magazine is on the shelves, and in it I throw up the idea why I think Domain Controllers might be your first upgrade. From the piece:

Microsoft's newest server OS will be ready by the time you read this. The manner in which you upgrade, though, probably won't be immediate. Here's my prediction: Your path to WS 2008 will happen faster than it did from Windows XP to Windows Vista, and upgrading your DCs will probably be your first move.

Think about the servers in your environment. Application servers typically have one or more third-party tools installed. This increases the likelihood of conflicts and complicates testing. Mail servers are exceptionally critical to your business, so upgrading these involves a lot of preparation and an equal amount of risk. Even file servers, often the least customized from the perspective of installed apps, are still cumbersome due to the potential for data loss.

On the other hand, DCs are the perfect storm of low customization, few installed apps and high redundancy thanks to Active Directory's peer-to-peer replication. The upgrade process is also relatively easy for WS 2008. Best of all, the benefits to your AD make it well worth the effort.

Read the rest at: http://redmondmag.com/columns/article.asp?editorialsid=2551