[tetonpete]

bet...you can safely fold to a raise. i seem to remember reading a theorem on here a while back stated that if you are heads up and the first to act, and a four flush card appears on the board, you should always bet regardless of your hand. (I might be misquoting this somewhat, but it surely applies here.)

you want a call from another ace, pocket pair or smaller flush, and you have a small chance of folding a bigger flush.