I am kind of amazed that that works. What's going on - does the baseband have its own IP stack, which it somehow shares to the computer, which then declares its own mini-lan and lets the iPhone on at a higher level such that iOS uses that preferentially?

That's bizarre; I'm tempted to try it out when I get back to a country in which I have mobile data.

"More happening in this iPhone, I feel, than has been revealed"

You can do that with a routing/forwarding table in any OS [that provides access to routing/chains]. Forward+NAT the cable tether to the 3G network, but keep the default gateway to WiFi, so the local applications will not use the 3G.

you don't need separate stacks to do this. each machine is the other's gateway, on separate ip addresses.

> Personal Hotspot

I always giggle at it being called that.