[ JDK VERSION : 1.6.0_07 ]

On Windows, user name is initially set from name of Window's current
user name. (LINUX etc.: I don't know)

IMO user name should be editable from Options->General dialogue.
It should overwrite templates user name, and maybe other values I'm
not aware about lacking deeper knowledge.

Workaround:
1) select Tools->Templates.
2) Expand User Configuration Properties, select User.properties and
open in editor.
3) Add e.g. "user=Mr. Unknown"

Additionally there also should be some project level user name, which
overrides it. There should be a reasonable relation to
application.vendor in project.properties.

Comment from Lukas Hasik:
"Uggh, that's ugly. ... IMO, it should be reflected in UI in better
way. Maybe a button "Change User Properties" in the Templates manager
or a special field in Options that would allow you to setup the user
name."