Rico.
I think there is a point when airlines begin to question whether it is correct to consider operating. BMI turned an aircraft around one day recently. I know that on the night the Gulf War (1) started BA had a 767 en route to Saudi which turned around somewhere over Egypt. They have to consider the safety of passengers, crew and the aircraft.
But... they are in business and when situations like Egypt and Tunisia occur there is business to be had and suprisingly it's two way. We've heard a lot in the UK about our citizens trying to get home. There have been a couple of stories about high profile individuals who have gone home to Egypt and there will be many less public people who will have gone home to Egypt and how do you think all the journalists we see (with their support crews, security men etc) reporting live got to Egypt?
Quite often demonstrations are surprisingly localised. For instance when the students had their day in London recently, or even the Poll Tax riot all those years ago, Heathrow, Gatwick etc didn't close. The same seems to be true in Egypt at the moment. The airport continues to operate.
Believe me, the airlines aren't going to take silly risks. The potential cost is too high.