According to a post in the Microsoft Surface Updating Software and Firmware forum at Microsoft Answers the 8.1 update for Surface RT devices, which was just released this past Thursday, has been pulled from the Windows Store.
The note comes from a user named Reagan_L who is a Microsoft employed Forum Moderator:
Windows RT 8.1. Microsoft is investigating a situation affecting a limited number of users updating their Windows RT devices to Windows RT 8.1. As a result, we have temporarily removed the Windows RT 8.1 update from the Windows Store. We are working to resolve the situation as quickly as possible and apologize for any inconvenience. We will provide updates as they become available.
We will certainly be watching this thread.
Have any of you experienced significant issues in your Surface RT upgrade process to Windows RT 8.1?