Might I add, that you can also use inflight Wifi for Skype or SIP phones in general! It's darn impressive, really.

On my last trip I even dialed back to a colleague who was also on a SIP phone with his laptop. (in the same airplane, but he could just as well have been on another one with Wifi)

The voice quality was definitely passable, with a slight delay and a few drop outs. Worst part was the ambient cabin noise through the microphone...