Jan Schampera recently pointed out that in struct _GmContact (in lib/gmcontacts/gmcontacts.h), there is an int state member which is documented like this :
  int  state;                   /* Status of the user:
                                        0: available
                                        1: do not disturb/busy
                                        2: auto answer
                                        3: forward
                                        4: called 
                                        5: offline
                                */

This should use a nice enum.