Microsoft will stop licensing XP to OEMs after January 31, 2008.
Microsoft will no longer sell XP through any channel after January 31, 2009.
Microsoft will continue full support for XP until April 14, 2009. After that, Microsoft will only issue security updates, nothing else, unless you're a corporation who has paid for an extended support contract.
Microsoft will discontinue all support for XP after April 8, 2014.
Of course this is all subject to change.
I will wait for Vista until I get a new computer and/or somebody hacks the DRM out of it.