PhilSpace
Where I feel for the chap who is now behind bars; there is surely more to this than meets the eye. I would wager that it has little to do with whether a flight plan is a secret document or not, and more to do with the Thai's wanting to get their hands on Shinawatra and the poor relationship between Cambodia and Thailand at present. Someone in his position would have to be extemely naive- at best, not to understand the potential consequences of revealing flight plan details of Thaksin Shinawatra's arrival in Cambodia to the Thai authorities.
The Guardian has a piece on the issues here.
Thaksin Shinawatra stokes tensions by taking job with Cambodian government | World news | guardian.co.uk
As an aside, I think I am correct in saying that all UK controllers and ATC people have to sign the Offical Secrets act, or at least they used to.