If iTunes switches, then everyone will use it ...

They use a different technique than in other lossy codecs (mp3, acc, etc.) without a psychoacoustic model. And the algorithms are actually quite old and not patented anymore (as in aac, mp3) - so in theory it should work ;)

However, I think libopus should also compile on the iPhone, so it should be possible to play .opus files on i-devices.

The codec itself is an entirely open standard, so no legal problems there. Whether there will be political pressure by major labels or Apple themselves remains to be seen. The benefits of Opus are huge, but then again, we got VHS, too.