3G Interfacing

Does a 3G shield currently exist for Arduino? If not is it practical to use a USB channel to send and receive via 3G by plugging in a USB modem? Or do they all use proprietary ways of communicating via USB (I notice many ones I have used requires software to be run to connect them formerly to the network)?

Would I be better off using a 3G Hotspot device and using WiFi to interface to it?

NO

NO

Well, that was certainly a useful answer. Which of the 4 questions asked does it apply to?

Does a 3G shield currently exist for Arduino?

Not that I know of.

If not is it practical to use a USB channel to send and receive via 3G by plugging in a USB modem

No because the arduino's USB connection is not a host so you can't plug peripherals into it like you can with a PC.
To do this you need a shield like this:-

But then you have to write the software - quite a daunting task.

Would I be better off using a 3G Hotspot device and using WiFi to interface to it?

Only better off in respect that you can't do the other things. A WiFi shield is quite expensive.

What are you trying to do?

I recently purchased a Parrot AR.Drone: http://ardrone.parrot.com/parrot-ar-drone/usa/
It runs off WiFi, and I have already made my own API to interface to it using the packets described in the system developers manual. I wish to interface to it via 3G (if the WiFi signal runs out of range). This is for a number of reasons but I suppose the most compelling one is range (and I am well aware of the latency issues here, but it's a compromise i would be willing to accept for the range), the other day someone else I knew who had the Drone lost it by flying it too high over an urban environment and letting the wind take it, a 3G backup interface would've been a blessing here. Being an iOS programmer I first wanted to simply put an iPhone into the hull of the Drone and interface to it that way, and take advantage of the iPhones GPS, magnetometer, gyro, accelerometer etc, however it weighs too much, so to get started I would like to create a bridge between a 3G connection and the onboard WiFi connection, and further down the track I would like to add Accelerometers, GPS, Magnetometers etc for more autonomous navigation.

Given the lightweight of 3G hotspots I don't see this as a terrible solution, especially if I get rid of the battery in them and make them use the Drones internal battery. However this would mean I would need 2 WiFi peripherals (presumably 1 for interfacing to the Drones WiFi, and 1 for interfacing to the 3G's).

Thoughts?

So can someone firstly tell me whether it's possible to put 2 WiShield 2.0's onto the same board?

You would have probs. with that because you have to have the drives and normally you have to install a software but you may be able to take apart the software for the data while useing the usb shield.