If you fly a Japanese airline, NH or JL, in a premium cabin, they'll offer you a mask as part of amenities. This has been offered irrespective of Sars. A mask perhaps helps you breath more easily in the dry air environment on the plane? I haven't flown JL's F since last year but you can get one of those honeycomb mask, which just covers up the nose. It is a bit funny to see in the F cabin all those mostly middle-aged Japanese guys, who seem to like them, wearing them to sleep.

CX provided me with a mask on a HKG-JFK a few days ago. Or rather an attendant insisted that I take one of those masks but not necessarily wear it. (JL and NH provide these masks in sealed plastic bags but CX just hands them out uncovered.)