While digging around in Hartmann Grisar, Luther V, I came across the following:
Of Mat. xvi. 18: "Thou art Peter and on this rock I will build my church," he gives the following explanation, which plainly rests on his own partisan and anti-Papal standpoint: By Peter all Christians together with Peter are meant, and their confession is the rock. "All Christians are Peters on account of the confession which here Peter makes, which also is the rock on which Peter and all the other Peters are built. The confession is common to all; hence also the name." "Werke," Erl. ed., 64, p. 194.
I was curious to see the context for this quote so I tracked down Erl 64:194. The comment is from a marginal note from Luther's translation of the Bible. I don't recall the complete set of these marginal notes ever being translated into English.
As to Grisar's charge of partisan and anti-Papal, it's too late to direct him to William Webster's The Patristic Exegesis of the Rock of Matthew 16:18.
Also while digging around I found my old blog entry where I nailed the Called to Communion folks on Luther giving the keys to Peter.