Microsoft has been saying that Service Pack 3 would be coming out mid-2008 but Neowin has managed to get their hands on the internal release schedule.
Microsoft’s Release Schedule is as follows
* April 14, 2008: Support is available for the release version of Service Pack 3 for Windows XP
* April 21, 2008: Original Equipment Manufacturers, Volume License, Connect, and MSDN and TechNet subscribers
* April 29, 2008: Microsoft Update, Windows Update, Download Center
* June 10, 2008: Automatic Updates
The large amount of time between releases is in case of some major issues in the Service Pack, which are bound to turn up.
Via NeoWin