A rad hardened machine won't be running 3.7, or 3.6.x for that matter. They will be running something a bit older with (guess I don't know for sure) security patches being back ported as needed.
I'd think that most of them would be using a RTOS like VxWorks or QNX.
Linux is used in a lot of places these days, where VxWorks or QNX might've been. I've been responsible for putting Linux to work in SIL-4 critical systems all over Europe, for example, saving millions in licensing costs while at the same time, getting Linux certified for safety-critical apps. It really is taking over.
Whats the current state of the real-time patches for linux? Last I'd talk to someone about them, this was a while ago, they felt a bit sub-par. From a developer's point of view is it as easy to work with and predictable as VxWorks?
Sure, its easy to work with. Just don't always try to work with the latest, bleeding edge kernels. Most RT/Safety-Critical Linux kernel installs are still based on 2.6.x, or even some cases, 2.4.x series ..