The alleged founder of Silk Road (explanation in next paragraph) earned approximately 600,000 bitcoins (~$75,000,000) and the FBI would like that money. The FBI arrested him last week and is trying to recover his money but they can't figure out his password and until they do, they won't be able to seize his alleged proceeds of crime.
Silk Road was a marketplace that operated as a Tor Hidden Service. It was accessible by using a special encryption program that allows people to browse the internet anonymously. Silk Road was used by many people to buy drugs from sellers around the world who then shipped them by mail. All transactions were done using bitcoin so that the sellers and buyers couldn't be traced.
How is it that the US government can't get the Silk Road founder's money? Bitcoins are stored in "wallets" that are encrypted using a private key. Doing any kind of transaction (like withdrawing the money) requires the key. So even if the government seizes the file that contains the information about the bitcoins (for which there are certainly backups) it's useless without the password.
In Breaking Bad (minor spoiler alert) the main character, Walter White, buries his millions in the desert to keep it from the government. If he'd been using bitcoins there'd be nothing to bury and a password would prevent the feds from getting his cash. Of course there are other questions about passwords, like whether the government can compel you to provide them (e.g. key disclosure laws, In Re Boucher, etc.).
Although there are issues with compelled password disclosure I think that Walter White (or any criminal) would prefer to have a file sitting on Dropbox that contains their millions rather than have to lug around barrels of cash.
There'll be more cases like this.