Ben Witherington has posted on his blog an article, entitled "The New Perspective on Paul and the Law" from his forthcoming book on NT Theology and Ethics, called The Indelible Image. I found his article pretty informative on his view of justification and the final judgment by which he sees moral apostasy as having a negative affect on someone's final salvation. Witherington writes:
"This is why the stringent warnings we noted about those Christians who could be excluded from the Dominion of God at the end for persisting in a certain course of disobedience such that they could be characterized as adulterers, thieves and the like, must be taken absolutely seriously. Final salvation, while it cannot be said to be caused by works of any Law in Paul’s thinking, can indeed be negatively affected in the end by persisting in sin such that a moral apostasy (or some other sort of apostasy) is committed, according to several key Pauline texts. All of this helps us to understand the ethical seriousness of Paul’s moral remarks and why he so often offers up such strong imperatives to his converts."
But I was perplexed in reading Witherington's understanding of Sander's, Dunn's, and Wright's view of the New Paul Perspective (NPP), especially as it relates to Judaism. Witherington seems to think that the proponents of NPP don't believe that Judaism required people to be obedient for salvation, but they just had to be in God's covenant in order to saved. He writes:
" In short, there are severe problems with the analysis of Paul in the New Perspective, whether we are thinking of the analysis of Sanders, Dunn, or even Wright. Paul believed that works and obedience in Judaism indeed affected righteousness, life, and salvation, the question is whether he carried such a belief forward into his Christian faith. If the old caricature of Judaism as a graceless and legalistic religion is certainly false, the New Perspective does not seem to have adequately represented the way Paul contrasts what is true in Christ and what he believed was true under the Mosaic Law."
I don't think this is correct. I believe NPP proponents have tried to maintain the balance of grace and obedience in their notion of "covenantal nomism". Dunn states:
"It is important to note...that Sanders did not characterize Judaism solely as a "covenantal" religion. The key phrase he chose was the double emphasis, "covenantal nomism". And Sanders made clear that the second emphasis was not to be neglected. The Torah/law was given to Israel to be obeyed, an integral part of the covenant relationship, and that obedience was necessary if Israel's covenant status was to be maintained. Even if obedience did not earn God's grace as such, was not a means to "get into" the covenant, obedience was necessary to maintain one's position in the covenant, to "stay in" the covenant. So defined, Deuteronomy can be seen as the most fundamental statement of Israel's "covenantal nomism". Given the traditional emphasis on Judaism's "nomism" it hardly surprising that Sanders should have placed greater emphasis on the "covenantal" element in the twin emphasis. But in his central summary statements he clearly recognized that both emphases were integral to Judaism's self understanding" 
HT (Denny Burk)
 Quote taken from Don Garlington's article "The New Perspective on Paul: An Appraisal Two Decades on". This is a must read article if you want a nuanced understanding of the NPP.