Syzygy - 10:07 pm on Apr 28, 2010 (gmt 0)
I'm not referring to public domain in the intellectual property sense, Tangor.
Public domain/public realm are also expressions in British English meaning that something is... made freely available to the public. You make up a joke and the instant you tell it to someone it's in the public realm.
You put out a viral message and you give it to the public to distribute as they please, without restriction.
Copyright would only apply to the unique nature of the content of the message. Change the content and perhaps an infringement may be considered. By its very essence a viral message depends on it being 'reproduced' time and time again. There is no infringement because the creator relinquishes their rights to control or restrict its distribution.