First press Win + R to get the run dialog where you have to enter "control userpasswords2" to get the dialog which you can see below.
In this dialog you have to uncheck "Users must enter a user name and password to use this computer." to achieve password-less login under Windows Vista (Home/Home Premium/ Business/Ultimate) as well as under Windows Server 2008 (Standard/Enterprise/Datacenter). After doing that the user selected in this dialog will be logged in automatically by the system as Windows starts. In this way you can speedup the Windows startup.
Of course this feature decreases local security (only) but significantly increases the convenience as it is with all features of this kind.
Update: Works in Windows 7 Beta as well.