It doesn't qualify as an in-terminal hotel, but perhaps interesting anyway: ARN has an old 747 that's been turned into a hostel. The upstairs is a lounge and the rooms, actually more like small cabins, are downstairs. And the flight deck has been turned into a suite. Great fun for any airplane enthusiast, but not very cheap for a hostel.
It's located near the Radisson Arlandia, 3 minutes on a shuttle bus from the terminals.