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.