I finally got hold of Sean Penn's publicist Mara Buxbaum to explain which overlooked actress he was referring to before he presented the best actress Oscar to Sandra Bullock Sunday night. "I never became an official member of the Academy," he said that night, "but the Academy and I do have in common that we neglected to acknowledge the same actress in our own ways two years running. So, I'm going to start fresh with the Academy and acknowledge these wonderful actresses."
Here's the answer:
It would be a reasonable assumption to say he was referring to Robin Wright since he didn't thank her last year in his acceptance speech, and the Academy failed to nominate her this year for The Private Lives of Pippa Lee.