Since today Software Update Services can deploy ServicePacks too! WinXp sp1 and Win2k SP4 are now synced into the SUS updatestore.
Think before approving this in your network, test if the SP completely works in your environment.
The packs will be distributed using BITS (background Inteligent Tranfer Service) so it should not fill upi your LAN if all clients download it from the SUS server, though it might take some time before all clients have the pack.
Welcome to Microsoft Software Update Services