I recently asked our Network Administrator if he could find out what he'd need to do to get our Cisco ASA 5520 to work with the built-in VPN client on Windows RT. Answer I got back:
From Cisco - ASA versions 8.4.5 and 9.0.1 and higher support the Microsoft Surface RT running Windows 8.
We are currently running version 8.4.4 so it will apparently be a no go at this point.
I don't have any more details, but hopefully soon our ASA will get updated to 8.4.5 and we are going to revisit this situation. Of course their comment of "Surface RT running Windows 8" is a bit confusing... but it sounds like there might be hope for the built-in client.