So I searched around for a while and stumbled across:
"Option™ GTM679W 4G EV-DO RevA with GPS + Wi-Fi Half-Size PCIe Minicard / Global Bands"
This is a unique WWAN/WiFi/GPS Mini PCI-Express card that weighs in at $130.
The description from the site:
What you see in the picture is just one part. It features WWAN connectivity (GSM/GPRS/EDGE + UMTS/HSDPA/HSUPA + 1xRTT/EV-DO RevA, 850/900/1800/1900 MHz), as well as 802.11g WiFi and GPS. 802.11n would have been nice but maybe there's a card out there that does it all that I just haven't found yet?
"Roam where you want to with this truly global cellular module which supports GSM/HSPA+ networks at blazing 14.4Mbps speeds! This half-size PCI Express minicard also has integrated GPS and Wi-Fi (802.11g) capability, making it one of the most advanced radios in the market! This product is ideal for any PCI-Express ready devices (Single board computers, notebooks, tablets, routers) where a single product SKU and 'carrier agnostic' applications are needed for worldwide deployment and manufacturing."
The data sheet mentions that the WWAN portion of the card has some kind of SIM-card interface, so it seems as though with the proper configuration one could connect an AT&T SIM (from an iPhone? an iPad? possibly with a micro-SIM to regular-SIM adapter?) or any other supported carrier SIM card and enjoy 3G + WiFi + GPS on the Air, without having to break out the soldering gun.
My only concern would be antenna placement and tuning: AFAIK the current Air models have a 2.4GHz-tuned antenna which would work with the WiFi card fine, but for the modem and GPS to work additional antennas would need to be placed somewhere in the Air.. and I've seen teardown pics, there isn't all the much space to play with, even for just wire placement, and I'm not really in-the-know when it comes to figuring things like best possible antenna configuration and how to minimize radio interference, etc.
Am I overlooking something obvious here? If this was possible and dead-easy, I'm sure others would have done a simple swap by now. Or is this as easy as swap-and-play? I've come out of the woodwork just to find what others think.