None of those products have mechanical click mechanisms. The front of the air is the thinnest part. It may be more of an engineering challenge than just shoving one in. The current button is as far as I recall slightly raised, whereas the glass trackpads are slightly recessed.Why? The iPhone, iPod touch and iPad all have (in essence) glass trackpads and they are quite thin.
Personally I think the Air's design is aesthetically almost perfect, it could just do with being a bit lighter.
Carbon Fibre would help here. If Apple have environmental concerns, then mix it with a natural fibre, like flax.