This was what I had in mind when I stated my concerns about non-published proprietary (or secret) "prior art". See my bold text below. This seems to imply that no publically available printed publications are required to prove prior art existed - the idea only needs to have been known or used in this country. But this part of the code may only apply to having a patent granted, perhaps not to challenges of an issued patent. Anybody know? 

http://www.tms.org/pubs/journals/JOM/matters/matters-9106.html

The prior art is defined by Title 35, United States Code, Section 102, which states: "A person shall be entitled to a patent unless...." This language is followed by a series of definitions, the most important of which are summarized in the following.

First, a person is not entitled to a patent if the invention was "known or used by others in this country, or was patented or described in a printed publication in this or a foreign country" before the date of invention by the applicant for the patent. If, for example, an invention is known or is being used by someone in the United States, another person who makes the same invention at a later date may not obtain a patent. Prior knowledge or use in a different country, however, is not a bar to a patent application in the United States. In contrast, a prior patent or a printed publication anywhere in the world will bar an applicant for patent in the United States if it appeared before the date of the applicant's invention.