Office 2003 SP3, released to customers on September 18, 2007, will become ready for automatic distribution via Microsoft Update (for Vista and XP) starting February 27, 2008. This means that the availability will happen gradually and not everyone will see it at the same time. The delay is there to give the market plenty of time to evaluate the service pack and allow the Office team to address concerns. The service pack includes improved security and enhanced compatibility with Windows Vista, Office 2007, Internet Explorer 7 and Windows SharePoint Services. According to the MU team, the customer feedback for SP3 has been generally positive.
News source: Microsoft Update Blog
This is probably the last service pack for Office 2003, so if you arent a fan of Office 2007, slipstream this service pack and burn yourself that new CD.