Microsoft has pulled the Windows RT 8.1 update from the Windows store investigating a situation affecting 'a limited number of users' updating their Windows RT devices, which is a big move for Microsoft. The exact issue has not been revealed but for Microsoft to pull the update means that the issue must be quite serious.
Microsoft states the following when asked by a user as to why the update was not in the store.
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.
There was no timeline on when the update would be placed back into the store, but if you updated your Windows RT device and it went smoothly, you may have dodged a bullet.