Yes, I have. On my last flight, in fact.

I boarded and took my exit row aisle. In the window seat next to me was a man hammering away on his crackberry...er...Blackberry. Not at all atypical for the situation of course.

A minute or so later, he tapped me on the shoulder and wanted to show me something on the screen of his Blackberry. He didn't make a sound, just motioned to the Blackberry. So I read it. He had typed "What time does the flight land?" So, I put up 6 fingers and said "about 6 o'clock" to which he mouthed "thank you".

A few minutes later the FA came around to give the exit row speech and asked if everyone understood. He didn't respond as he was typing away on his Blackberry to someone else. The FA tapped him and asked him if he understood and he pointed to his ear and shook his head. The FA then said very slowly that he would have to be moved. She found a volunteer to take his seat and they switched. No other problems.

That was my first time seeing a deaf person on a flight...that I know of.