Identity Theft
From the US Department of Justice website (http://www.usdoj.gov/criminal/fraud/idtheft.html#What%20Are%20Identity%20Theft%20and%20Identity):
While this may be a crime in the United States, [someone] believes that very few other countries specifically target the identity theft as a crime in and of itself, so unless the perpetrator is within the jurisdiction of the United States (or Canada), prosecution on this charge alone will be very difficult. The summary seems to suggest that identity theft is only a crime if the theft is used within the course of committing another crime.
Most countries have laws against using another's identity for financial gain in their fraud laws. However, UserName spoofing on the wiki (the original reason for this page) does not fall within the definition since there is no financial gain involved. TombStoning is much more serious, especially since we can much more easily defend against UserName spoofing (though imperfectly).
Username spoofing may result in action under LibelLaws.