I've done it before as a consultant. I billed it to my company and they paid it with no hesitation. It turned what would have been three hours of downtime while I go to visit one client into a few hours of productive communication and some amount of work with several clients during my traveling. When the company is charging clients $150/hr for my time, the $10-$20 it costs to pay for wifi on a flight more than pays for itself if I can bill even a single hour while I'm on a plane.