Is this before or after you have extruded? I noticed that on Maya 5, I needed to activate an option in the Channel Box, called 'Keep Faces Together'. It defaulted to off, and it needs to be on.

What you need to do is select the faces of the wing object that you have got from converting the NURBS object, and select the Extrude Face tool. A number of options should appear in the Channel Box. Scroll down until you find Keep Faces Together. Where is says Off, change that to a 1, and it will change to read On. Then just extrude as normal to add the thickness.

If this is before you have extruded, it is because the polygon object that you have created is flat, therefore it doesn't have a bottom. Extruding the faces adds the bottom faces, and moves the top faces up (or the other way round).

Hope this helps.