I was told by the Revenue Department, just last week, that if you pay tax on the money you bring into Thailand, by Law, you will still be required to submit a Tax Return, together with a Certificate from your home country confirming that you have already paid the Tax.
When I asked how they would know about money coming into Thailand they said that you would also need to submit a 12 month Thai Bank statement.
They went on to say that there WILL be additional checks om ALL foreigners. They are currently in discussions with both Thai Banks and Immigration.
The above is NOT scaremongering, it is a genuine report of my conversations with the Revenue Office.