That would be rather difficult to prove. Conversely, it'd be difficult to prove that you didn't forget the password. I think it comes down to a Fifth Amendment issue. If a judge decides it's not protected under the Fifth Amendment, then I suppose you could be held in contempt of court.

http://www.brettwmartin.com/contempt-of-court-in-colorado/co...

How could you prove that I didn't forget it? It's impossible.

It's impossible to prove it beyond a reasonable doubt, but there could be a preponderance of evidence that you didn't forget it. For instance, someone could testify that they saw you log in the day the computer was seized.

http://en.wikipedia.org/wiki/Legal_burden_of_proof

However, you could argue that you didn't log in using your PGP password that day, but rather your OS level password. Your habit is to never shut down your computer, but rather to let it stand by, so very rarely do you ever use your PGP password. Earlier that week you changed your PGP password, but you can't remember it. The password went something like blah blah blah. This way you are cooperative, without turning yourself in.

It would be akin to say "this safe? I lost the keys.", wouldn't it? I don't know what's the standard way of dealing with that either though.

Depending on what was inside of it, they would just have someone open it.

I imagine with enough willpower or strength any safe can be broken into. Not so much with encryption.