Most people are not switching to Windows Vista so Microsoft is setting up deadline for Windows XP support. From the press release:

Today Microsoft announced the worldwide extension of the availability of Windows XP Home for an emerging, new class of mobile personal computers commonly known as ultra-low-cost PCs, or ULCPCs. Windows XP Home for ULCPCs will be available until the later of June 30, 2010, or one year after general availability of the next version of Windows.

This is not extension for normal XP or XP business version. This is for computer such as Asus EEE Pc.