The official release is January 30. OEMs are getting the RTM version now to prepare for the official launch. Most OEMs are offering free upgrades to Vista for purchases before January 30. XP Pro gets you Vista Business, XP MCE gets you Vista Home Premium and XP Home gets Vista Home Basic. If you want to build your own machine you can get the upgrade coupon by purchasing an OEM licence of Windows XP.
http://www.microsoft.com/windowsvista/