My friend Josh Edwards is quoted by CNET talking about SP3.

Service Pack 3 for Office 2003 will be focused on security, said Joshua Edwards, a technical product manager for Office at Microsoft. "We're trying to take what we learned from building Office 2007 and bring as much as we can to Office 2003," Edwards said in an interview with CNET News.com.

Microsoft hasn't yet set a release date for the Office 2003 update, which like other service packs will be available as a free upgrade. Also, there are no details of what will be in the update, other than that Microsoft is "backporting" work it did for Office 2007.