If you have clustered Exchange 2003, you need to get a fix (for free) from PSS. For more information about this read this article:
http://support.microsoft.com/kb/841561
As for the rest of you wondering whether to install Windows 2003 SP1 on your Exchange 2003 servers the answer is yes and no. Exchange servers are primary targets for security attacks, so installing such an important security update should be top priority. It has also had extensive test runs through its beta and release candidate cycles. Those cannot cover the entire Exchange 2003 base nor can they specifically address your server. If you can, test in the lab. If this is not feasible, make sure you can roll back the service pack installation. A backup of the operating system might also be a good idea because sometimes all you need to roll back is a single DLL.
You can also wait a while, tune in to feedback on the Internet. Don't be discouraged by some negative reaction because there is always someone with a failed service pack installation, which in a lot of cases is a problem with third party drivers. It might be a good idea to call your hardware and software vendors and ask them if they have tested the service pack in the release candidate cycle.
