According to The Guardian, which was one of the chosen publications by WikiLeaks to help spread the information, the cables were copied from the defence department's Siprnet network by a 22-year-old soldier, Bradley Manning, who was working at an army base near Baghdad.
Manning's been held in solitary confinement since April, after his unauthorised downloads were discovered. According to the soldier, it was all too easy to gain access to the network and download the files:
"I would come in with music on a CD-RW labelled with something like 'Lady Gaga' … erase the music … then write a compressed split file. No one suspected a thing ... [I]listened and lip-synched to Lady Gaga's Telephone while exfiltrating possibly the largest data spillage in American history."
Amounting to 1.6GB of data, Manning claims that "information should be free. It belongs in the public domain", however he made the mistake of IMing another hacker, Adrian Lamo, and boasting of his deeds. Lamo then dobbed him in, while Manning uploaded all of the files to WikiLeaks.
Gawker has a run-down of some of the more "interesting" factoids contained within the cables, but really, none of it is as horrifying as the video of an Apache helicopter firing at Baghdad civilians, which is believed to have been obtained by Manning and released by WikiLeaks back in April. [The Guardian]