It may be most familiar to some through its use in American films and television dramas, but this practice is by no means limited to the United States. According to Wikipedia, "usage of 'John Doe' originated [in Great Britain] during the Middle Ages ... the legal term John Doe injunction or John Doe order has survived in English law and other legal systems influenced by it."
Wikipedia goes on to quote an academic document published by the University of East Anglia: "If an unknown person has possession of the confidential personal information and is threatening to disclose it, a 'John Doe' injunction may be sought against that person. The first time this form of injunction was used since 1852 in the United Kingdom was in 2005 when lawyers acting for JK Rowling and her publishers obtained an interim order against an unidentified person who had offered to sell chapters of a stolen copy of an unpublished Harry Potter novel to the media."
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