What FCP is actually doing is sending it out 'as-is'. In that... you shot anamorphic which is really 4:3. What FCP ISN'T doing is taking the anamorphic flag from the sequence and incorporating it into the final export. Why? BEATS ME!!! I've been on Apple to do that for 9 years!!!
SO... you can try opening the file in QT and hit APPLE-J. Click on the NAME OF THE MOVIE CLIP and then the PRESENTATIONS tab. I think CLEAN is what you want. I never do it that way. If I'm exporting it in full rez DV it is usually going somewhere else where I'll force 16:9 (compressor or iDVD or by using....)
CONVERSION:
Once you choose your export type there will be an OPTIONS button. Click that and then SIZE. Force the 16:9 from there... 480x270 etc.