Dave Warlick attributes the term 'Personal Learning Network' to George Siemens's 2004 paper, Connectivism: A Learning Theory for the Digital Age. In the comments I add that the term 'Personal Learning Network' is directly derived from 'Personal Learning Environment', which as this history shows was first used at the The Personal Learning Environments Session at a JISC/CETIS Conference in 2004 (this comment has yet to be approved by moderators as of this writing). Warlick wonders why he is being asked about the origin of the term: it could be because he has been busy popularizing it, as in this ISTE magazine article. For an overview of the concept of the PLN, see Lucy Gray's new slide show on the topic, and this presentation from David Jones on PLEs. Also, one of the most interesting PLNs I've found out there is this one by Chris Smith.