Microsoft confirmed this morning that it has temporarily removed the Windows RT 8.1 updater from Windows Store on Windows RT systems because of an issue.
Microsoft provided the following statement.
"Microsoft is investigating a situation affecting a limited number of users updating their Windows RT devices to Windows RT 8.1," a Microsoft representative said. "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."
More info as it becomes available.
Update: Surface RT users who borked their devices trying to upgrade to Windows RT 8.1 can revert to the original version of Windows RT using as discussed in Surface RT 8.0 Recovery Image.
Update 2: Microsoft has provided the following additional statement on Monday, October 21, 2013:
Based on our investigations of a situation customers have encountered updating to Windows RT 8.1, we can confirm that as of now this is a Windows update issue only affecting Surface RT customers. While only less than 1 out of every 1,000 ( or less than 0.1%) Surface RT customers who have installed Windows RT 8.1 have been impacted, improving their experience and ensuring their systems are fully operable as quickly as possible is our number one priority. We have made recovery media available for download along with actionable guidance for affected customers. We continue to work towards making the Windows RT 8.1 update available in the Windows Store again and apologize for any inconvenience. Further updates will be provided as they become available.
Update 3: On Tuesday, October 22, 2013, Microsoft made the Windows RT 8.1 upgrade available again. I'll have a separate write-up available soon.