I was thinking the same thing. Where is the proof that these are in fact “real”?
Because you haven't had anyone in the administration come out and say that it's fake. In fact, you've actually had official memos (see post just above yours) disseminated to all government employees warning them not to read the still classified cables on the Wikileaks site. It's an explicit administration acknowledgment that they are indeed real. Plus, you have foreign diplomats resigning because what they're quoted as having said in some of those cables.
This is no guarantee that what is yet to be released, will be genuine. But, it's pretty clear that what has been released thus far, is authentic.