MS has pretty standard schedules on which they "discontinue" software, and it's not short. MS would like to stop selling XP now, that's likely to happen when Windows 7 ships, unless there's a commercial reason not to.

As for support, wikipedia says "Windows XP Service Pack 2 will be retired on 13 July 2010, almost six years after its general availability ... On 14 April 2009, Windows XP will begin its "Extended Support" period that will last for 5 years until 8 April 2014"