The big Windows 8.1 update will be available to the public on October 18th. An entry on the Windows Blog this morning confirmed the date and gave more information.
"I know a lot of folks are eager to find out when they will be able to get Windows 8.1. I am excited to share that starting at 12:00am on October 18th in New Zealand (that’s 4:00am October 17th in Redmond), Windows 8.1 will begin rolling out worldwide as a free update for consumers on Windows 8 through the Windows Store. Windows 8.1 will also be available at retail and on new devices starting on October 18th by market. So mark your calendars!"
Link: Windows Blog