I've interop-unlocked my ATIV S.
The process is a little hokey, and relies on having the Samsung disagnosis app; we'll either need to find a way to get that onto other phones, or find a similar vuln in them.
It's possible, though, and I've done it. Stock TELUS Mobility ROM, standard AppHub dev unlock (should work with the free unlock).
I can install apps with both ID_CAP_INTEROPSERVICES, and with non-public capabilities (internal or private), and they run.
This is a beautiful, beautiful day. It's 5AM and pouring rain outside and I need to be at work in four hours, but I could hardly be happier!
Real thread forthcoming. For the moment, I need to dance around a little and see what else I can do with it.
Win8/Windows RT projects:
List of desktop apps for hacked RT devices
Native Access WebServer and Libraries
WP8 Interop Unlocks
Storage Cleanup tool
XapHandler, Root Webserver, OEM Marketplace XAPs, Bookmarklets collection (Find On Page), Interop-unlock hacks. Do not private message me with questions that should have been posted on the forum!
Not only are you wasting your time - I'm not going to bother writing an answer to such a question for only one person - but I will probably block you from PMing me in the future as well.