It's interesting to see the way rival newspapers have covered Nick Davies' allegation that the Observer's former political editor Kamal Ahmed helped write the Februay 2003 (really) dodgy dossier. See the Standard website.
The Sunday Times includes the claim in a story by its arts editor, "War Stirs Left's backstabbers", while the Independent also tells the story as a feud between the pro-war Observer and the (slightly) anti-war Guardian.
I've known about these claims for a while and, while I don't really know if they are true, my experience of Ahmed is that he was one of the journalists most open to being fed a story during the Campbell era. It wouldn't surprise me at all if he returned the favour.