Some nondisclosure agreements (NDAs) contain a provision stating that all disclosing party information is presumed confidential unless proved otherwise.
If the disclosing party were to file a lawsuit for misappropriation of confidential information, like any plaintiff it would be required to prove all the elements of its claim. This would normally include demonstrating that particular information is confidential, usually by showing that the disclosing party has made reasonable efforts to keep the information confidential.
A presumption-of-confidentiality provision, however, if it were enforced by a court, might well reverse that burden of proof: It could force the receiving party to prove that the information came within one or more exclusions from confidentiality.
As a practical matter, the receiving party is going to put on whatever evidence it can anyway to show that the information was not confidential.
But a presumption clause might be deemed to shift the risk of nonpersuasion to the receiving party.
(For a useful summary of evidentiary presumptions, see Paul R. Rice, E-mail, I presume: Authentication law needs updating for the e-commerce age, Legal Times, week of Jan. 22, 2001 (accessed Oct. 17, 2008).)