Thai Bitcoin exchange, Bitcoin.co.th, has published a notice on its site saying it is ceasing operations indefinitely due to a ruling by the nation's authorities: Starting tomorrow, Bitcoins are illegal in Thailand. While the ruling will apparently be revisited, but there's no timescale for that on the table.
The Foreign Exchange Administration and Policy Department is said to have decided that, because there are no applicable laws that can control or tax Bitcoins, they are illegal. Buying them, selling them, buying goods or services for Bitcoins, selling things for Bitcoins, and transferring Bitcoins across the nation's borders is forbidden.
Essentially Thailand's government has decided that since it's mechanisms are too old-fashioned to regulate, tax, or otherwise profit from Bitcoin cryptocurrency, it's not going to allow it. Interestingly, there's no prohibition on mining Bitcoins, which is the actual creation of the currency units themselves. That could mean Thai residents can make them on powerful PCs locally, but not trade in them, at least until the government catches up with the tech.