The following is an excerpt from an article on FCFighter.com:

UFC Welterweight Champion Georges St-Pierre has signed a six-fight contract extension with the organization, according to the fighter's manager Stephane Patry. St-Pierre previously had two fights left on his old contract with the promotion when negotiations began, and with this latest signing, the previous contract has been replaced with a new six-fight deal. In an interview this afternoon on FCF Radio, Patry also stated that talks with the UFC had been difficult at times, but that finally an agreement was made that would make St-Pierre "probably one of the highest paid fighters in the UFC, probably top three." Patry declined to comment on any further specifics regarding the contract's finances.
I'm a little surprised by this deal. Granted GSP is one of the best 170lb fighters in the world, but the UFC usually likes to sign 3 fight deals or less. I guess they expect him to hold on to the belt for a long time and they don't want him leaving with it.