I don't think a firm date has been set as of now. I am guessing sometime in Q4 2010 (or possibly early Q1 2011).
"... the merger is subject to approval by a majority vote of the outstanding limited partnership units of NRGP, which is expected to be held in the fourth quarter of 2010. Certain members of management that control over 50% of NRGP's limited partnership units have committed to vote in favor of the merger. "
<<<and at what price are holders of NRGP shares converted to NRGY shares? >>
They are converted at a ratio not a price. You get .77 shares of new NRGY for each share you won of NRGP. The new price will be whatever Mr. Market says they are as of that day.
"Under the terms of the merger agreement, NRGP unitholders will receive 0.77 NRGY common units in exchange for each NRGP common unit they own at closing, representing a premium to NRGP unitholders of approximately 10% based upon the 20-trading day average closing prices of both securities ending August 3, 2010. "