Always, except when I am close enough to the ground to get a signal. As others have noted, it doesn't make sense to drain the power by searching for a signal.

I enjoy the feeling of sending a surreptitious text message after takeoff or on approach. However I would be loathe to use voice services, that would just seem to be asking for trouble with the flight crew and the signal tends to fade in and out.

I do wish carriers would stop with the "safety issue" nonsense.

Curious - with the planes that now have wi-fi service, how do they deal with skype?