The 16:9 setting on a clip in Final Cut Pro is entirely artificial -- for example, you can check the "Anamorphic" box on a regular clip and FCP won't know the difference. What the 16:9 marker on a clip means is that when the clip is edited into a timeline, it will behave differently -- if you edit a 16:9 clip into a 16:9 sequence, the clip will remain stretched height-wise. This stretched version *is* what 16:9 looks like. If you edit a 16:9 clip into a normal sequence, the sequence will read the marker on the clip and do the letterbox crunch for you. This means that when you output a movie file from this sequence, your footage is *no longer in 16:9*.
16:9 footage is supposed to be stretched -- so that monitors, TVs and DVD players with a "16:9" mode can then crunch the image on their own.