I'm pretty sure that it's ckpool on top of the p2pool backbone.
There were problems using ckpool as a proxy with heavier network loads. Right now NastyPool uses the P2Pool software to delegate to miners. Since then ck does much work to improve proxy mode.
NastyPool's NastyPoP payout method would like to use ckpool as a P2Pool sub-pool frontend. When I find some time I will give it another test.