I believe the *City* of Jerusalem is not mentioned in the Qur'aan, in the same light, that the Qur'aan does not mention any other cities. As far as I know, the Qur'aan only states the name of a tribe or a group (i.e. people of 'Ad, people of Yathrib etc).
Just because a certain city is not specifically named, it does not render it completely irrelevant.
One has to note, that a city name may be changed as time progresses. Before Palestine was occupied, the area now known as Jerusalem/Yerushalayim was previously named Al-Quds (and might be named differently 1400 years ago). Some with any other cities in the world. With different rulers/empires, city names would have undergone changes.
It is foolish that such and such names are not mentioned in the Qur'aan...