This information is preliminary and based on the SP Roadmap.   

Article: Windows XP SP3 Pushed to 2008
http://www.betanews.com/article/Windows_XP_SP3_Pushed_to_2008/1161282900

QUOTE: Windows XP SP3 will be the first major upgrade to the operating system since XP SP2 debuted in August 2004. SP2 was an extensive upgrade, bringing a new security center and improvements in wireless networking and Internet Explorer. However, with SP3 arriving three years later, the update will focus security patches and bug fixes rather than feature enhancements.

Microsoft's SP Roadmap
http://www.microsoft.com/windows/lifecycle/servicepacks.mspx

QUOTE: SP3 for Windows XP Professional is currently planned for 1H CY2008. This date is preliminary.