In Leopard Apple decided to switch the default Thai font from Lucida Grande to Thonburi, making the Thai characters in the former inaccessible. This causes problems because a) the Thonburi Bold font is non-functioning for some reason and b) Thonburi's spacing is botched for mixed Thai/Latin text.
Pending Apple's fixing these issues, one idea is to replace Thonburi in System/Library/Fonts with a different Thai font. I downloaded the Garuda set described at this site and used FontForge to rename them Thonburi. After making backup copies of Apple's Thonburi, I replaced it with the renamed Garuda, and this seems to work. A copy of the renamed set can be gotten from my iDisk (the folder Garudathonburi). Feedback on whether this solution is helpful would be welcome.
Another fix would be to replace the Lucida Grande in Leopard with the same font from Tiger. I have seen reports that this does not seem to cause any problems.