I came across a French blog posting by David Sebban (presumably from Microsoft France) at MSDN Blogs in which he mentions that the beta for XP SP3 is now available for 12,000 invited testers (mainly manufacturers and retailers?) and that a public beta will be available soon.
David also mentions that XP SP3 will not contain any new functionality except for the addition of the NAP (Network Access Protection) Client which has been known for quite awhile.
Following is the translated text via Google:
Microsoft, Windows XP, Security Update, Windows Update, Microsoft Update, XP Update, Service Pack, XP SP3, Beta, Public Beta
In the movement of the “vagueness 2008” which will contain SQL Server 2008, Visual Studio 2008 and Windows Server 2008, 2 Packs Service will leave in the neighbourhoods the first quarter 2008: the SP1 of Windows Vista and the SP3 of Windows XP.
This last is available besides since today for 12000 invited beta tester (mainly of the manufacturers and the retailers) and will not contain new functionalities except the addition of customer NAP (Network Access Protection). This customer is necessary if you wish to integrate working stations XP in an infrastructure NAP Windows Server 2008.
The SP3 of Windows XP will thus contain mainly updates of safety existing and some new hotfixs and beta public will be available soon.