[sflamm]

I'd like to see an app that configures the Iphone as a NAT/Gateway.

Imagine a simple on/off toggle in the control panel.
When toggled "on" it would:

1. Create an ad-hoc network of a specific name or join one (private subnet)
2. Act as an IP router (use the built-in routed daemon)
3. Route (forward) all traffic from the private ad-hoc network (i.e. 192.168.x.x) to the active network (wifi, 3g/edge)

You can do this easily on most linux boxes by setting:

net.ipv4.ip_forward = 0

Combine this with a few "route add xxx" rules and you have a generic router...

I have found this discussion thread on the same idea:

[Project] iPhone as a 3G router - Page 2 - Hackint0sh