I would like to caution the TR members that Service Pack 1 for Windows 2003 has several well-documented bugs listed in Microsoft Technical Article 89367:
http://support.microsoft.com/kb/896367
I would caution everyone that has Exchange 2003 and Citrix in the environment that this service pack will be extermely disruptive.
Currently there is a known issue that remote Citrix Xp FR3 clients cannot connect. Microsoft recommends contacting Citrix, which is a finger-pointing gesture at best and a cop-out at worst.
Exchange 2003 SP1 has several well-documented issues such as when running in mixed-mode XML attachments are blocked in Outlook Web Access, Privacy issues are caused when you use Simple Mail Transfer Protocol (SMTP) addresses for mailbox addressing in Outlook Web Access and MIME Encapsulation of Aggregate HTML Documents (MHTML) attachments display differently in Outlook Web Access.
Service Pack 1 also changes the authentication for DCOM and RPC, which could lead to issues in applications not using an account in the computer-wide Access Control List (ACL).
My advice on this Service Pack is to begin testing but delay implementation in a production environment for a minimum of 1 quarter and 2 quarters is my personal recommendation to clients. This service pack is essentially a very deep re-write of the kernal and services.