Is the use of a trademarked name as a username
problematic?
I don't think so, because it is not associated with the sale of a good or
service.
The relevant US code states:
http://www4.law.cornell.edu/uscode/15/1125.html
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(c) Remedies for dilution of famous marks
(1)
The owner of a famous mark shall be entitled, subject to the principles
of equity and upon such terms as the court deems reasonable, to an
injunction against another person's *commercial use in commerce* of a
mark or trade name,...
The following shall not be actionable under this section:
(A)
Fair use of a famous mark by another person in comparative commercial
advertising or promotion to identify the competing goods or services of
the owner of the famous mark.
(B)
*Noncommercial* use of a mark.
(C)
All forms of news reporting and news commentary.
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That may not stop stupid people from sending nasty letters, but I think we
can safely ignore them.
Regards,
Erik