IANAL, but the "search incident to arrest," for all its broad powers, still doesn't reach the strength of what a court can compel you to do.

During arrest, you have the right to remain silent.

It's a good question, but legalisms aside, having your phone securely encrypted with a strong passcode gives you much more control over whether your phone is going to be decrypted.

Even if there's a circumstance in which the police can compel you to decrypt your phone, you're virtually certain to be able to have your attorney present when that happens.

The current precedent seems to indicate that no one can make you reveal your passwords. ASFAIK, the EFF was fighting a similar case on these grounds.