It's a myth but a myth that has long been circulated & believed. Why?
Because "sometimes we expend a great deal of time, effort, and money to create a "high-tech" solution to a problem, when a perfectly good, cheap, and simple answer is right before our eyes." - http://www.snopes.com/business/genius/spacepen.asp
The real story is more about innovation & entrenpeaneurship than anything else... http://en.wikipedia.org/wiki/Space_Pen