'High priority' for Windows XP & Server 2003, 'important' for Vista & Server 2008
Starting the third week of April (April 20) Microsoft will begin pushing Internet Explorer 8 to Automatic Update / Windows Live Update for users of IE6 and IE7.
It will initially be pushed out to a small user base, and slowly expand the distribution to all users. Users will see the update listed as a 'high priority' update for Windows XP and Windows Server 2003 and as an 'important' update for Windows Vista and Windows Server 2008.
The update will not automatically install, instead users will have to opt-in during the welcome screen.

Network administrators who do not wish for IE8 to be installed need to use the (free) Blocker Toolkit, which was made available in January. Microsoft plans to release IE8 to Windows Server Update Services (WSUS) sometime in July.
If you want to help, tweet on twitter when you receive a notice from Windows to update to IE8 and tell everyone your XP/Vista, country, IE6/7 and we can track the rollout together.