The best interpretation I've thought of after watching it (and talking with my wife for hours after) was that Saito is the architect and he wanted to get back at Cobol Industries twofold:
1) By getting Cobb to convince Fisher Jr. to feel like his daddy loved him and wanted him to be his own man and dissolve the business to start his own empire and
2) Get back at Cobb for trying to steal his secrets for Cobol by _incepting_ the little seed of "you want to go back home to your kids" in his head and making that his obsessive ends to justify all means. Saito, meanwhile, was willing to wait the dream equivalent of _eighty years_ to exact this revenge. Then, as DiCaprio fulfills his promise and gets Saito in his 80-year Limbo, Saito fulfills his promise, architects a layer where Cobb does reunite with his still-young-as-Cobb-remembers-them children, and the spinning totem wobbles but doesn't actually fall, then the "I N C E P T I O N" credits roll, because Saito ran the inception on Cobb.
I'm not sure that the Inception on Fisher Jr. ever actually occurs, as I doubt that Cobb ever escaped Saito's dream from the beginning.
It is suggested in the movie that the subconscious attacks the architect of the dream first. In the beginning of the movie the last we see of the first architect is him being bundled away by Saito's goons.
Saito then crafts an excuse for Cobb to end up at peace, lost in his own subconscious. The Saito living in limbo could be a projection in the same way that Fisher Jr. creates a projection of the guy the forger was mimicking.
1) By getting Cobb to convince Fisher Jr. to feel like his daddy loved him and wanted him to be his own man and dissolve the business to start his own empire and
2) Get back at Cobb for trying to steal his secrets for Cobol by _incepting_ the little seed of "you want to go back home to your kids" in his head and making that his obsessive ends to justify all means. Saito, meanwhile, was willing to wait the dream equivalent of _eighty years_ to exact this revenge. Then, as DiCaprio fulfills his promise and gets Saito in his 80-year Limbo, Saito fulfills his promise, architects a layer where Cobb does reunite with his still-young-as-Cobb-remembers-them children, and the spinning totem wobbles but doesn't actually fall, then the "I N C E P T I O N" credits roll, because Saito ran the inception on Cobb.
Ta-da!