David Arquette -- 1 Year Sober, Parties with Courteney Cox

Proving they are the happiest separated couple in Hollywood, David Arquette celebrated his first full year of sobriety by eating dinner with Courteney Cox at Mr. Chow in Beverly Hills last night.

Arquette had checked himself into treatment on January 1, 2011 to deal with "alcohol and other issues."

As for the marriage ... the couple separated back in October 2010 ... but never actually filed for divorce.