When if involves a criminal defense. That's the beauty of our criminal defense system. You're innocent until the government proves you are guilty. The government cannot withhold information it has, that may help prove your innocence, even if it involves national security.
Sort of. The Jabara case is a nifty example of how the government can assert state secret without queering the entire trial. The defendant is entitled to the most exculpating inference that the withheld data would provide, if shown.
http://www.freerepublic.com/focus/f-news/1550960/posts?page=238#238 <- scroll UP for more