There have been a number of useful discussions of the two kingdoms doctrine of late and I thought it might be helpful to post links to them (in case you hadn't seen them).
First up is Kevin DeYoung's discussion of the pros and cons of the two kingdoms approach to Christianity and culture, as well as that of neo-Kuyperianism.
The White Horse Inn blog offers responses from Jason Stellman and Darryl Hart here (Click here: Why Two Kingdoms? » White Horse Inn Blog), and here (Click here: More on Two Kingdoms » White Horse Inn Blog).
Darryl Hart weighs in on the specious argument that the civil kingdom is neutral ground because it is common ground. Click here: Old Life Theological Society » Blog Archive » Neutrality, Schnootrality