This is what I'm hoping. But there was a very similar feature a month ago (which didn't actually do that) so I'm not holding my breath.. I want it so I can bring my Apple TV to locations with no wifi and airplay netflix from my unlimited iPhone data plan... This would have come in handy just a few weeks ago... But no, my iPhone can't even feaking use the remote app without being on the same network...
I believe i found a work-around for the problem you're describing.
My friend wanted to stream from their iPhone to the ATV but didn't have a wifi network setup at their home yet.
I suggested setting up a hotspot on the iPhone and having the ATV connect to it.
It worked for me at my home, hopefully it may work for you too!