While Microsoft has been rather coy about the actual release dates for Windows Vista Service Pack 1, Jason over at techAU says he received the following believable schedule:

4 February
SP1 available to OEMs (HP, Dell, Acer etc)

1 March
SP1 Volume Licensing availability (MVLS)

18 March

SP1 available on Windows Update, Download Centre, MSDN & TechNet

18 April

SP1 pushed via automatic download