It isn't only Hersh (WaPo, some named sources)
A Washington Post article with what appear to be a wider range of sources says much the same thing -- that planning is underway and nuclear strikes *are* being considered. Ken Pollack (Brookings analyst) is quoted as saying that nukes are being discussed, and as someone who knows Ken and the sort of contacts he has inside, he's unlikely to be talking out of his ass. The WaPo is not quite as dramatic or certain of which way decisions are headed, but it's consistent with Hersh.
http://www.washingtonpost.com/wp-dyn/content/article/2006/04/08/AR2006040801082.html?nav=hcmodule