This is being tossed around right now in a case in Colorado involving a woman accused of running a mortgage scam. A laptop was found in her bedroom during a raid by police. Declan McCullagh at CNET reports that the woman and her attorneys are arguing that to undo the encryption on the laptop would violate the 5th amendment, protecting the accused from being forced to be a witness against him or herself. Prosecutors, with the backing of the Department of Justice, say that unlocking the information that's in there should be considered part of the standard discovery process that happens in every trial.
Much of the discussion has been about what analogy comes closest. Prosecutors tend to view PGP passphrases as akin to someone possessing a key to a safe filled with incriminating documents. That person can, in general, be legally compelled to hand over the key. Other examples include the U.S. Supreme Court saying that defendants can be forced to provide fingerprints, blood samples, or voice recordings.
On the other hand are civil libertarians citing other Supreme Court cases that conclude Americans can't be forced to give "compelled testimonial communications" and extending the legal shield of the Fifth Amendment to encryption passphrases. Courts already have ruled that that such protection extends to the contents of a defendant's mind, so why shouldn't a passphrase be shielded as well?