French President Nicolas Sarkozy has submitted plans for his country to rejoin NATO after a 43 year absence. His reasons are not to benefit NATO, not to add to the common defence of its members, not to aid NATO in its worldwide commitments, but to make sure that France has a platform to exert its proper influence over world events. This isn't a guess on my part, he's flat out stated it in public and to his own government.
Charles de Gaulle withdrew France from NATO in 1966, for two main reasons. First, he was upset by foreign powers (the US) basing nuclear weapons on French soil with no French control over them. Second, he was upset that France was not the dominant force behind NATO decision making. His decision mandated the removal of all US military forces from France and its possessions, causing a large effort to move and rebase forces in the middle of the Cold War.
France has cooperated (more or less) with NATO in the years since, but has not had forces under NATO military control. Sometimes cooperation has been good, sometimes not ( i.e. French denial of overflight rights for US military aircraft, interference with US action against Libya, outright sabotage of US efforts in Iraq).
Discussions for the rejoin have been going on for a couple of years, and have been endorsed by NATO's leadership, several NATO countries, and the US.
France has participated somewhat in NATO's operations in Afghanistan.
Sarkozy faces stiff opposition from the opposing French Socialist party, which had been in power until Sarkozy's election. France's wish to join must also be ratified by the member NATO countries, though that seems to be a certainty.