As for the capability (now expired) to stop Windows XP SP2 being delivered through Microsoft Update, now a similar toolkit is available for download from Microsoft.com to block SP1 for Windows Server 2003. It is critically important to realise that the blocking capability is temporary. After 30th March 2006 (1 year from SP1 release), the block will no longer be enforced.
Although I've been using SP1 for several months now both in beta and RTM releases and found it to be rock solid, this tool may be important for you and your organisation while you continue to evaluate the use of SP1.
For more information on how to use the toolkit which is essentially in three variants (an executable to set a registry key, a script to set the same registry key, and an administrative template which can apply the registry setting through Group Policy), click here.