Microsoft have temporary removed the Windows 8.1 upgrade for Windows RT from the Windows Store. They say there are investigating issues affecting a limited number of users and have removed the upgrade.
I upgraded my Surface RT without any issues and I know other people that have upgraded without any issues.
Hopefully the upgrade will be back soon.
UPDATE:You can download recovery images from Microsoft but they seem to be US only.
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.