The official Microsoft Internet Explorer blog reports that IE8 is almost ready and that a blocker tool is out already to help IT admins / corporate customers block the install until they have a chance to test IE8 on their network:

The IE8 update will be released as the highest priority update for each operating system. For Windows Vista and Windows Server 2008, it will be listed as Important. For Windows XP and Windows Server 2003, the update will be listed as High Priority. Delivery of IE8 via AU will begin after we make IE8 available from the Microsoft Download Center. Of course, users can always decline to install IE8 through AU when it is offered. More information for IT Professionals about IE8 delivery via AU is available here.

If you previously used the IE7 Blocker toolkit to block IE7 from being offered as a high-priority update, you will need to run the IE8 version of the Blocker Toolkit to block IE8 from being offered via AU. There are different registry keys used to block or unblock automatic delivery of IE7 and IE8. If you configure the IE8 Blocker Toolkit setting to prevent users from installing IE8 via WU/AU, IE8 will not appear in the list of available high priority or important updates. We believe this approach strikes a good balance by helping customers become more secure and letting organizations control when they are ready to deploy IE8 to their users. Note: The IE8 Blocker toolkit will not block the final version of IE8 being offered to users who already have pre-released versions of IE8 installed on their machine. Also, the IE8 Blocker toolkit will not prevent users from manually installing IE8 from the Microsoft Download Center.