I'm currently reading Elaine Pagels, The Gnostic Paul, and I can highly recommend it. @Laura
briefly refers to this book in FPTM as well.
Pagels basically reconstructs the Valentinian/early Gnostic reading of Paul. Now, my hunch about what went down is that the Valentinians were indeed a direct successor of Paul's "school" if you will, probably with a direct lineage (Valentinus claimed his teacher was a student of Paul). But as it always happens, the Valentinian teaching was corrupted somewhat over the years; there apparently were different factions/splits that we don't know much about, etc. In particular, their thinking became a bit too elitist: thinking in terms of "the elect" and "the psychic". They probably also over-emphasized the idea of predestination, i.e. that the "elect" are predestined to "know God". They also read later Gnostic concepts like the Sophia worship into Paul (or perhaps there was something there that is lost).
Nonetheless, their reading of Paul emphasizes the gulf between "those who live according to the flesh" and those who live "according to the spirit", and to my mind they are right on the money here. They read Paul symbolically, and seem to have a good grasp of the "unseen world" and its relationship with the world of the flesh.
One idea in particular is fascinating, although I'm not sure yet if the early Gnostics didn't exagerate it a bit: they claim that Paul deliberately worked two layers into his written works, namely a "fleshly" Gospel that appeals to those "of the flesh", and a deep symbolism that appeals to those of the spirit. That is, he foresaw the twisting and misunderstanding and destruction that would ultimately set in when those with no eyes to see try to interpret true spiritual teachings, and for that reason, he produced the fleshly gospel about Jesus Christ the man and the bodily resurrection, talked about the importance of the Law, and so on.
Now, perhaps these "fleshly" parts had been injected into the Pauline corpus by that time, and the Gnostics weren't aware this, so that's just how they tried to make sense of it. Or they didn't understand the true conflict between Paul and Peter, the Pauline groups and the zealots, and interpreted this conflict exclusively in symbolical terms.
Still, the idea that there are "two Pauls" so to speak would go well with the Markan gospel as both an allegory for Pauline teaching and a "codification" of it for those who "live in the flesh".
I mean, Christianity has certainly persisted, and despite all the corruption and nonsense still has preserved some crucial teachings. Whereas the Gnostics are gone, probably after they lost much of their initial truth and descended into "fleshly" thinking and sectarianism themselves... So IF that was Paul's plan, or at the least the plan of his successors, it seems to have worked to some extent at least.
Anyway, some very interesting stuff in that short book. Even though it claims to be about theology and not history, some likely historical development may be inferred, especially about the fate of the Pauline teachings in the second century and after.