Since 2010, there might be one (big) change :
Going from the french side to the swiss side of the airport is not possible unless you have a boarding pass.
I don't know how strict this is enforced nowadays but be careful.

Source :
https://www.gva.ch/en/Site/Passagers...0-999c98a2877e
Warning: Following security measures imposed by the Swiss and French authorities, Genève Aéroport informs you that ONLY passengers in possession of an air ticket valid for day D +/-1 are authorised to pass between the international sector and the France sector, and vice versa.

Therefore, car rental agencies’ clients that do not meet this condition must imperatively move from one sector to another by road (car, taxi, etc.), by passing through the Ferney-Voltaire customs. Pedestrians and cyclists are not allowed to use this road.