Following in the footsteps of peers that include The Big Show, Trish Stratus, Lita, Chris Jericho (twice), Jeff Hardy (well, it was his original plan), and Batista, CM Punk has not yet renewed his WWE contract and looks to be leaving the company voluntarily to take a break when his current deal expires in a few months. As you'd probably expect, WWE wants him to sign a new long-term deal.
Being burnt out (he's been at or near the top of the lists of the wrestlers working the most matches in the company each year for most of his run on the main roster) and dissatisfied with how he's been booked for the last year or so, he's ready to rest up for a while. While he's done a strong job with whatever the creative team has thrown at him for a while now, WWE doesn't seem fully invested in him as a top level guy, even though he did well during his World Heavyweight Championship reigns, is regularly main eventing house shows,and is in the mix with top level opponents like John Cena and Randy Orton. A source for that one spyware site noted that WWE may use his departure as an excuse to not push him.
With Edge's retirement, it'll be interesting to see if WWE gets desperate and makes some kind of big offer or just go with the status quo, which would seem to guarantee Punk's departure. Given his burnout, even the former wouldn't necessarily come close to guaranteeing that he stays since he has saved his money well.