The MacBook Air's single USB port is a finicky, tight fit, and lots of stuff—like a lot of 3G modems—just can't squeeze into the space, which is a big deal for mobile-focused notebook. Om Malik had already ordered his Air before that revelation came out, so he tried to make do with Novatel's U727 USB modem for Sprint's network. And surprisingly, it fit, "without problems."
Ryan at Engadget said the discrepancy could be because "he's forcing his in a little further, or his trap door has a little more vertical give," since Ryan tried out a second U727 as well.
According to Om, the bitch w/ the U727 is on the software side, because "some 'genius' at Apple decided to hide the Internet Connect option in the Macbook Air version of Leopard," so it takes some poking to get it going, plus a manual connect each time. Bottom line, though: "Buy this modem to get the most out of your Macbook Air—US$279 for a new one, US$79 with a 2-year contract from Sprint." Whatever 3G modem you want/have/get, be sure to test it on a real machine first to avoid clenched fists and squeezy eyes.
AU: I know this isn't actually relevant, but if you're planning on getting a MacBook Air, you may want to think about how to go about mobile broadband locally after this revelation.