Could it happen again? Well the US seem to think so in light of the Manning and Snowden affairs, and there have been recent press reports saying how they are really clamping down on financial issues and debt.
Interestingly BBC4 have just shown a really good 2 parter on submarine warfare in the Cold War. They explained one of the biggest leaks from the Walker family was so damaging in enabling the Soviets to catch up in the technology stakes that the Akulas were nicknamed Walker Class boats. Seems he did it for money too, so I guess they have been burned in the past and therefore their current approach is understandable.
It might be interesting to see if people turn traitor for different reasons in different countries - is there a national stereotype?