Just heard this reported on New Hampshire Public Radio News at 11AM. They said only that it was due to a campaign issue and that the party would have until next Wednesday to name a replacement.
Just found this confirmation http://www.seacoastonline.com/news/6_11special.htm
Nothing on the state party web site or the Cohen web site with any more info.
I personally would hope that Jeanne Shaheen might step up to the plate. She could beat Judd Gregg and has the name recognition - plus the ability to turn on the money.