Windows RT, the ARM version of Windows 8 is set to be released on October 26th and with in Microsoft is expected to include the new tablet optimized version of Office. However Office 2013 will not be ready in time for the Windows 8 launch and Microsoft will be releasing Windows RT with a Preview version of Office 2013 instead.With Office being the biggest selling point for the Windows 8 tablet, it was important for Microsoft to include it with the release of Windows RT. However the Preview release of Office 2013 for Windows RT will not be a full version. It will lack macros, VBA support, third-party add-ins and a few other minor features.
However Microsoft Office 2013 is expected to be updated to the full version once it is ready for public consumption sometime in 2013.
source - TheVerge