I certainly agree with you that "the OS" doesn't just mean the kernel nor even the basic userland anymore.
Surprisingly, it was the legacy BlackBerry OS that made me realize this. It has seamless IPC too, but only as a side effect of a horrifically insecure and unstable memory model.
There have been many attempts to build this sort of capability into mainstream operating systems, but none have become popular due to the inescapable complexity of IPC. A recent example is D-Bus, which is supposed to be "lite" yet still involves XML interface descriptions and serialization. It's enough of a hassle to implement that nobody bothers, unless it's the only option or they are evangelizing the technology.
QNX is built from the ground up to do everything this way. If it makes IPC easy enough, it could bring this type of software architecture to critical mass.
I realize this is still pretty vague, but I can't think of a specific example at the moment. If you're interested, read up on QNX, or Coyotos, which is a more experimental OS that is based fundamentally on IPC.
Also, writing off QNX as just another Unix is a mistake. An RTOS with synchronous IPC baked in to the kernel is potentially game changing technology, though I am skeptical that RIM will manage to fully exploit it.