Previous leaks have been blamed on Bradley Manning who worked in a communications role in Iraq, however it is very hard to describe a plausible explanation to show how he might have come into possession of information that would reasonably be communicated directly between US embassies and the US State Department.
We seem to have another source.
Thus far, the full body of material (some 251,287 documents according to Wikileaks) has been given to a small number of news outlets and only a tiny subset of 220 documents made generally available.
The CableGate site promises a steady release of all material over the coming days / weeks.
Additionally, the same report indicated that a Denial of Service attack had been launched against Wikileaks by someone with the handle the3ester (The Jester) and as this article was being written, the main Wikileaks site was initially shown as a simple holding page, but in the last hour or so, the 'normal' material returned.
This suggests that the site has been reconfigured to avoid the DoS attack.
Wikileaks and its founder Julian Assange have had a somewhat chequered past with allegations of rape in Sweden, of financial difficulties and problems with donors actually being able to contribute to the cause.
There have even been rumours of involvement with children's privacy.