Over on the Ligonier website, Jason Stellman has written an article over the necessity of Reformed theology in embracing the Covenant of Works. As he wrote,
This idea that the original Edenic covenant was a conditional covenant of works, and that Adam had to fulfill its conditions in order to gain the stipulated reward, can be a jagged pill to swallow for many Reformed folks. After all, doesn’t this all sound so legal? Where’s the grace in all of this?
Ah, but this is precisely the point! As I mentioned above, the legal nature of the Edenic covenant actually protects the graciousness of the gospel that would be proclaimed later on.
To make the pre-Fall covenant in Eden and not of works is the error of all Mono-covenantalists. Whether they are Barthians, Federal Visionists, or PRCA makes little difference — the only difference is how consistent they are in bringing their denial of the Covenant of Works to its logical conclusion.
In the best case scenario, even if it was possible to somehow totally disconnect Adam and Christ such that one can somehow say that "Christ merited salvation for the elect through His works" while denying that Adam was under a Covenant of Works and thus denying the typology between the two (cf Rom. 5:12-18), the issue of the Fall then becomes a big problem. Why did the Fall happen? If it is agreed that Adam transgressed a law, and the Edenic covenant is supposed to be a gracious covenant, then law and grace are not antithetical to each other. Salvation by grace therefore is not antithetical to the idea of a condition of staying in the covenant or "state of salvation" by works/obedience/faithfulness. This results in the error of what I generically call Monocovenantal Legalism.
On the other hand, if it is denied that Adam transgressed a law, then the Fall is either not real or not sinful, perhaps a natural development of growth like what the Gnostics taught. That is paganism.
There is however a third way of trying to solve the problem, exemplified in the PRCA. That is to agree that Adam transgress a law, but then to say that transgression does not break the covenant because the covenant is defined as a relationship or a bond of friendship and love, and is thus unconditional. Therefore, while Adam sinned and brought death into the world, yet the covenant relation between God and His people is not broken and cannot be broken. This error I generically call Monocovenantal Antinomianism, which we will see why.
In this scheme, the horror of the Fall is minimized. This is not to say that the PRCA treats the Fall lightly, but because it is treated as being merely quantitatively different from post-lapsarian sin and iniquity. Whereas in the Reformed system, the Fall is qualitatively not just quantitatively different from post-lapsarian sin and iniquity. Adam sinned under a Covenant of Works in the Reformed scheme, whereas for the PRCA Adam sinned under a gracious covenant.
This brings us to the related doctrine of Creation. If God can only relate to creatures on the basis of grace and wrath (ie there is no "neutral" state), then Man must be created in constant need of grace in order to live in God's favor. The patristic and medieval (and Roman Catholic) idea of donum superadditum would be utilized here. Grace perfects [defective] nature, not renews it.
Both of these would tend to trivialize sin. Instead of seeing sin as alien to nature, it is taken to be an evil that pertains to nature. Those within this "covenant" (as defined by the PRCA) therefore tend to minimize sin as being natural while still fighting it as part of the nature versus grace contest of the ages. Such tendencies are what make the system considered "Antinomian", not necessarily in practice but in doctrine.
Having rejected the biblical idea of covenant, it will be indeed interesting to see to what extent the PRCA will move towards Eastern Orthodoxy with its similar idea of covenant as relationship. While certainly more needs to be read up, it seems to me that the destination of Monocovenantal Antinomianism does not seem to be Roman Catholicism as much as Eastern Orthodoxy.
 See for example Peter Toon's description of Doctrinal Antinomianism, in Peter Toon, The Emergence of Hyper-Calvinism in English Nonconformity, 1689-1745 (London, UK: The Olive Tree, 1967). Available online at http://www.anglicanbooksrevitalized.us/Peter_Toons_Books_Online/History/hypercal1.htm. Accessed on Jan 3rd 2011.