There's something else worth noting on the message at Wikileaks -- they have added something new:
Secondary verification by google.com DKIM key
This is a big deal, and I wish there was an explanation, or at least a link to an explanation.
What this means is that the message was signed with DKIM when it originated at GMail. Neera Tanden sent the message from her Gmail account, and the DKIM signature was added to the header at that time.
What does this mean?
The message is authentic, and has not been modified since origination.
The DKIM signature is generated with a mathematical algorithm, using the content of the message and selected header fields as the input. In this case, the Date:, From:, Subject:, and To: fields are among the header fields verified.
More information here: https://en.wikipedia.org/wiki/DomainKeys_Identified_Mail
Why is this important?
The participants cannot claim the message has been "doctored" to make them look bad.