While Windows XP will no longer be sold in retail stores after June 30, 2008, Microsoft has publicized details on just how long (and how) they will provide support to customers with Windows XP in this article:

Mainstream support delivers complimentary as well as paid support, free security updates and bug fixes to all Windows customers who purchase a retail copy of Windows XP (i.e. a shrink-wrapped, not pre-installed, copy). Mainstream support for Windows XP will continue through April 2009.

Extended support delivers free security updates to all Windows customers. Customers can also pay for support on a per-incident basis. Extended support for Windows XP will continue until April 2014. New bug fixes require the Extended Hotfix Support program.

Takeaway: While Microsoft continues to market the advantages of Vista, they also have clearly committed to issue security updates for Windows XP through April, 2014. This is good news for home and business users who want to get the most out of their investment in Windows XP.