I don't think the uranium story is invalid and I don't think the administration said it was invalid. What they said was that given the CIA can't independently confirm the uranium story, it should not have made it into the State of the Union address. They further explain (clumsily) that the British and others stand by their sources and the story. It's a minor difference.
I feel the administration could have done a better job explaining the situation, but given that the liberal press has misquoted them so many times, perhaps they DID DO a good job and I'm just not able to hear their explanation very well through the liberal filter!