The "Mythology of Jesus" theory was an obscure and generally dismissed theory.
It has gained exposure due to author "Acharya S" and her collaboration with the Zeitgeist filmmaker. (She is also author of the films companion guide)
Some other views:
" The idea of Jesus as a myth is rejected by the majority of biblical scholars and historians. In 2004, Richard Burridge and Graham Gould stated that they did not know of any "respectable" scholars that held the view today. Robert E. Van Voorst has stated that biblical scholars and historians regard the thesis as "effectively refuted"."
http://en.wikipedia.org/wiki/Jesus_myth_hypothesis
"There is no shortage of information which refutes this viewpoint. But this type of conspiracy relies on two elements common to web surfers: First, they're only willing to check facts at the most superficial level, which usually means they'll read a message or watch a clip just once before forming an opinion and moving on. " Ref:
http://media.wildcat.arizona.edu/media/storage/paper997/news/2008/01/28/Opinions/Internet.Idiocy.The.Latest.Pandemic-3171363.shtml
"...(a) common problem in the film: presenting something in such a shallow manner without further corroboration or scholarly evidence." - Ref:
http://gauntlet.ucalgary.ca/story/12284
For a collection of rebuttals, check out:
http://webskeptic.wikidot.com/zeitgeist-story-of-jesus
Other links:
http://www.boingboing.net/2007/08/06/jay-kinney-reviews-z.html
http://en.wikipedia.org/wiki/Jesus_Christ_in_comparative_mythology
http://en.wikipedia.org/wiki/New_World_Order_%28conspiracy_theory%29