Now that he's at the point of no return with a UFC contract, it depends on how that turns out. If he can get some respectable wins and sell PPVs for UFC, I think WWE would entertain bringing him back and give him strong booking. Lesnar pissed a lot of people off, but after becoming a UFC champion and making money for them, WWE saw the value in bringing him back and he's the champion now. I'd expect a few obligatory "penalty jobs" same as Lesnar, followed by a similar mega-push.
Now, if he comes back after having his ass handed to him and washing out, it's a different story. I think his star power will sink if that's the case and a lot of his die-hard fans will probably even bail on him. No monster push, because WWE will look at it as no big deal. If Lesnar is still on the payroll even on a part-time basis, Punk won't have much to boast in the way of credentials. It's a risky endeavor for him, at any rate.