If someone is willing to divulge inside knowledge of his last employer you have to assume that in the future he will be willing to divulge your inside knowledge.

Agree. But it happens all the time. Why else do you think they have those long periods of gardening leave when you leave one employer for another, or those clauses in contracts saying you will not work for a competitor in the same field for X months when you leave? (Although, I seem to recall the last one being unlawful since they are essentially stopping you earning a living).

Sometimes employers are willing to take that risk.

Do you think Ed Snowden is now more or less employable now?

His options are certainly more limited now (considering most employers could not keep him safe). That being said, I imagine his existing options would be readily available and would all pay millions.

Very true. Trust is important -- hard to win, easy to lose. Once lost, it never comes back.

I am pretty sure they would be considered a traitor if someone told their new company how the NSA was doing things, just look at what they are calling Snowden.

Yes, absolutely, I agree with you. But just because you were ex-NSA does that mean you can no longer work in related work? What if you built their data centres, and you took a job at Google and tasked with building their new data centres. Do you just "forget" what you already know? I understand you may be under NDA to not discuss your previous work, but that doesn't meaning you can't make suggestions or plans without explicitly spelling it out, esp if your employer knew your background and you may not be able to speak about it openly.