Sgt. Longhorn, granted, that said "Curveball's" information was considered suspect at best by the German's. When that information was passed along it was prefaced as such. The Niger/Uranium story had been discreditted by the Nigerians, the Italians (the original source), the British AND the US prior to Powell's arguements. That pretty much leaves the aluminum tubes, which was open to interpretation at best.
The reason why intelligence agencies insulate their sources from the president is to guarantee that the information is properly vetted or at least offered as suspect. When the Bush administration began to bypass the normal proceedure and began "stovepiping" information, they put themselves in a place to cherrypick intellegence without having to address credibility. Whether they believed the threat was credible or not was immaterial, as the information they were looking at was considered flawed even by the nations which originated it.
Powell by taking the administration's word that the information was valid, set himself up for a fall. As such, yeah, I think he is too damaged to have a political future going forward. He can still be viewed as an American Statesman, but what he did in front of the UN was political suicide.
