Universal 3G, $200 rebate with 2 year contract
3G modem that will be built-in into future apple portables will be multi-band, the same way iPhone is. But will be based on GSM for slower speed in the areas where no 3G available.
Apple will offer an optional contract with Mobile carrier (AT&T in the USA), and it will make it $200 cheaper.
It will not be limited to MacBook Air, it will be available in all portables, yet not all at once.
3G tethering from iPhone to the computer via USB or Bluetooth or even Wi-Fi would be a good option to consumers, but carriers do not want it.