I knew about this already, but still it's interesting reading.
There's actually a system of similar purpose in British service, known as the Last Resort letters. These letters from the PM are placed in a safe on each Vanguard missile submarine. In the event that the UK ceases to exist, the letters are opened and the crew follow his or her instructions, which are probably one of four options.
1. Take out Moscow (or whoever)
2. Contact the Americans or the Australians (if they're still around)
3. Do nothing
4. Use your own judgement.
I'm sure other nuclear powers have their own systems.