Yesterday, a download referring to the service pack appeared on Microsoft's Web site. The download, "Office 2003 Service Pack 3 Administrative Template (ADM), OPAs, and Explain Text Update" only includes policy settings and documentation, but more could be on the way.
CNET reported back in April that another service pack for Office 2003 was forthcoming, but didn't give any guidance as to timing. Back then, a Microsoft official told CNET that SP3 would include security updates and potentially features included in Office 2007. I'm checking with Microsoft to see if I can find out any more.