Oprah's Big Job Send-Off Raises Big Fashion Question

So you have resigned from your job where you have worked for 25 years. Fittingly, they decide to throw you a big send-off and invite some special guests. They book your home town's biggest venue (like the Sandton Convention Centre or the Good Hope Centre, for example) and they keep most of it a surprise.
You arrive and the place is full of people who have loved you for all these years, people who continue to admire you, respect you and aspire to be you. And then your farewell speeches start and people come onto the stage to sing to you and there are tears. This is what it must have been like for uber-boss Oprah. And these were the special guest speech and performance people:

Gayle King (of course), Maria Shriver (yikes - but looking amazing), Aretha Franklin (seriously), Jerry Seinfeld, Tina Fey, Steve Carell, Josh Groban. Patti LaBelle, Usher, Jamie Foxx, Stevie Wonder (seriously,  again), Will Smith, Jada Pinkett-Smith, Michael Jordan (I've met him!), Tom Hanks, Tom Cruise, Katie Holmes (looking cute, if crumpled, in silk shorts), Halle Berry (in Victoria Beckham), Queen Latiefa (stunning), Beyonce (fishnets for performance, elegance for stage), Madonna (in what looks like Prada or Miu Miu) and Dakota Fanning (probs Valentino). Not. Joking.
Could there have been a more stellar line-up?
But the big question is, who designed that gorgeous purple, silk crepe embellished frock that Oprah wore? It really suits her and flatters her figure like few dresses I have seen on her before. My money is on either Rachel Roy or Temperley London. Now over to you. I will put a request in for the real answer - you guys let me know who you think the designer is. Good game! Now, go.
UPDATE: My boss, Samantha Page, editor of O magazine, got hold of Adam Glassman, the fashion editor on O magazine in the US and he tells us that Oprah's divine dress is by L'Wren Scott. Thanks Sam. Mystery solved.