Georges St. Pierre vs Nick Diaz superfight now possible after UFC 129

Not only is it possible, it's damn near the only option for Georges St. Pierre at 170-pounds.
That's because he effectively cleaned out the welterweight division last night, April 30, 2011, by defeating Jake Shields at the Rogers Centre in Toronto at UFC 129.
Shields essentially represented the last credible challenger in the UFC that doesn't make his home in a different weight class. However, he is NOT the last credible challenger in the division. There's a certain other Cesar Gracie fighter, who resides under the Zuffa banner, that has long felt he could take it to GSP.
Nick Diaz.
So could the fight ever happen? Here's Dana White's take on it:

"I imagine I could do whatever I wanted (with Nick Diaz), if I really wanted to but we have a contract with Showtime and he's a Showtime fighter. So, I don't know, we're going to have to see how this whole thing works out. But (Georges St. Pierre vs. Nick Diaz) is an interesting fight. Jake (Shields) said he'd like to see it."

Interesting may not be the right word. Necessary might better describe it.
And what does Shields have to say about all this?

"Of course I would love to see Nick get it, he's one of my best friends and I think he has a good shot. I think his style matches up pretty good. His pressure boxing could hopefully do what I couldn't and cut him off and put more pressure on him. We don't know what happens until we see it, that's why I'd like to see it."

Diaz most recently pasted Paul Daley on April 9, stopping "Semtex" via strikes in the very first round.
His "pressure boxing" game, which relies on a mixture of volume, power and relentlessly pushing forward, could very well give St. Pierre trouble.
He would also have the advantage of one of his teammates having basically scouted GSP for an entire five-round, 25 minute fight.
Of course, there are obstacles, mainly Diaz's professional boxing aspirations and his current status as a Showtime fighter, as Dana White noted.
But, as he also stated, he can more or less do what he wants. And that just might include putting together the very first crossover superfight since the purchase of Strikeforce.