I believe that they also told the Guardian that the password was "time limited", whatever that means. There was probably some melodramatic chat about it self destructing in 10 seconds. I don't see The Guardian as being on the hook for this.
I don't know how you'd time limit the password itself. It isn't as if I could send you an encrypted file, with a key that will only decrypt that file for that day.
Even if Assange had made absolutely sure to destroy any copy he had of the file which used that key (which would have been a Good Idea), he could not guarantee that some router on the internet didn't store a copy of the entire download session.
Regardless of how the blame is distributed, the decision to publish such a sensitive password in such a public manner still stands out to me as the most boneheaded single action in the entire saga.
Assange didn't want to give them access to the cables at all, but relented when they demanded it in negotiations with him. He then made them sign a contract to protect the information [1]. Clearly this wasn't enough though.
[1] http://wikileaks.org/IMG/pdf/Guardian_Letter_for_Package_3.p...