Just for a little accuracy (for the benefit of Sky News/AP), the flight was about to depart Phoenix NOT land. "Hundreds" would have been the 300 or so pax on board, I guess....
It had left the gate but had not reached the holding point (and it's a very short taxi). Very acrid smoke was coming from the rear of the passenger cabin (galley, maybe??) and some young children were carried forward because of this smoke. I understand the smoke had something of a peroxide-type smell.
It seemed a little while before the decision was taken to deploy the slides (though perhaps it was not) and some main deck passengers were directed to the upper deck slides because it was taking a while to clear the main deck forward door areas (the Y class cabin at least appeared completely full or almost so).
The latest local news says no actual fire was detected by fire crews. Usual grazes etc, from the slides, but nothing serious as far as I know.