Sastira writes "Jordan Bunnell has successfully modded his Macbook Air to use an internal EVDO Broadband card. In order to make everything fit, he had to sacrifice the WiFi and bluetooth cards, but considering the portable nature of the laptop, it is a small price to pay for being truly wireless."
(You'll need to scroll down past the sketchy-looking top section of that page.) Sastira also links to this InsanelyMac forum thread
on the mod.