I was on a WN flight many years back (HOU-PHX) (when the seats faced each other), and with the help of the crew and an annoucement over the PA, a guy proposed. She said yes, and the whole plane bursts in to applause. The girl totally ate it up, and I could tell there was quite the anticipation to get to PHX and then...., but I would agree it depends on how your gf will respond.

Let us know how it goes...