Marco Arment says:

If Apple wants to offer 4G in MacBooks, they can start whenever they want. Doing it properly will just take a bit more effort than adding a modem.

They can’t do it whenever they want. It requires new regulatory approvals as well as the much harder testing and certification with carriers (domestic and international). This would be required for each MacBook model.

Perhaps the Google Chromebook Pixel will force Apple’s hand to building it and dealing with all the red tape. Like Marco, I’d at least love for connection APIs to be in place. When tethering, I don’t relish that Software Update might be downloading big updates or Xcode is grabbing large docsets.