"Starting tomorrow, the installation will be fully automatic with no user action required for those who already have Automatic Update enabled. SP1 will be released gradually over the coming weeks to all customers on the RTM version of Windows 7. The service pack will take slightly longer to install compared to other updates. To ensure Service Pack 1 is installed without issue, customers should check for sufficient free disk space and that AC power is present on a laptop. If additional space needs to be created, we recommend using the Disk Cleanup tool to delete some files so that the service pack will install. If the service pack installation is interrupted, it will reattempt to install automatically after the next restart."
This automatic update will be applied only to Windows 7 consumer PCs that are not managed via other Microsoft management tools like System Center Configuration Manager or Windows Server Update Services (WSUS), the post noted. Some admins have chosen to block temporarily their users from getting SP1 until they've tested it to their satisfaction. Those in that category won't be getting SP1 pushed to them starting tomorrow.