I'm currently enjoying a rather wonderful Martini, found a good article on them at Salon.
On the olive, lemon zest discussion, it really has to be lemon.
If you leave the olive in till the end, the martini tastes overly strongly of olive, otherwise you have to eat it. If it is a poor quality olive you are left with the taste of olive all through your mouth and martini.
Hence Lemon zest, nice, citrus refreshing and constant.