As far as I can tell, you can't have your iPhone on both a Wi-Fi network and 3G network at the same time. The iPhone uses one or the other, not both.
Not true; I used to have my old iphone 4 jailbroken with mifi. when i had the tethering on, and also wifi switched on, it would automatically have the little wifi icon at the top. i was successfully able to use my laptop and my phone on the internet at the same time, even on verizon 3g.
I also have quite a bit of experience with my express, as long as the wifi is ready to go when you plug the express in, it connects right away and remembers its settings.
So put these together: make sure the tethering or whatever you have is broadcasting wifi before you start your car, and the airport will connect no problem, and the iphone should pick up airplay within about 30 seconds. i am actually really interested in this idea and will try it, as soon as the untethered jb comes out for A5 devices.
And for those of you who think this is pointless when you can just plug it in cause you're 2 feet away, you're right. BUT, how cool would this be just to say you can do it?!
Only complaint i'd have tho, is how long could you use 3g, wifi, tethering, airplay, and other stuff without having to charge your phone? cause once it's plugged in to the charger, you just defeated the whole purpose of this idea: to go wireless. i suppose you could put a powermat thing with the powermat case in your car, but now it's gettin ridiculous lol. Just my 2 cents, but good luck anyhow
