Why would they threaten that? An encrypted laptop would be useless to them without the password. The worst they could threaten you with is denied entry or detainment, so I think it's far more likely that they would threaten with that to compel you to produce the password.

In any case, if you are worried about protecting your data, you put yourself in a pretty bad situation if you rely on encrypting that data and hide the password. Bringing a relatively clean laptop will just avoid the issue altogether and will allow you to be cooperative with them.

Remember that they don't _need_ any cause to deny you entry or detain you, as long as you're not a US citizen.