As eber42 already pointed out, there's not much of an app to write -- that functionality is already present. One more reason I'm glad to be running an slightly-updated PDP-11 SDK on my handheld computers.

I could be mistaken, but I believe I've seen detailed instructions for setting up sharing a BT DUN link over wifi in Maemo 4; that should stay almost entirely the same for sharing the built-in connection on Maemo 5. I haven't messed with it, but IIRC you did need to run a custom kernel.