Partial Source: Pwinsider
Here is the latest on CM Punk appearing in the corner of Chael Sonnen at UFC on FOX 2. Sources say that this is not a deal that was put together by the UFC and WWE, instead, it was more of something Sonnen and Punk put together on their own. There is no promotional plan in place between the two companies, and this is not something so do with setting up a Brock Lesnar return to WWE.
UFC on FOX 2 takes place the night before the Royal Rumble, and there is a possibility that WWE could get a plug out of Punk's appearance for the PPV. Not many people within WWE knew about the appearance until it leaked out on MMA sites.
It would not be impossible for the two companies to work together. In 1997, WWE licensed footage of Ken Shamrock in UFC to educate their audience as to who he was. Also, when Brock Lesnar debuted with the UFC, the company licensed footage of Lesnar from WWE for commercial use. Shane McMahon, who is no longer with the company, put that deal together.