Hi anne-mie, and welcome.
As you already know, I can't help directly, but I have sent a message to our #1 complex script guy, and hopefully he will check in soon. I didn't want you to think nobody noticed you or that we don't care if it takes a little longer than normal to get a good response.
Funny, I just finished a Thai job in CS5.5 - that also included Lao and Burmese, and another dozen or so languages. I've not had access to the ME edition from Winsoft in quite some time, and that's the major difference between your installation and my installation. So I can't tell you why Thai is failing in CS5.5ME.
CS4 + World-Ready Composer = correct rendering of Thai, no bad line breaks
CS5.5 + World-Ready Composer = correct rendering of Thai, no bad line breaks
CS4 + Adobe Paragraph Composer = incorrect rendering of Thai, pretty much guaranteed. Is there no way for you to try it in CS4 with the World-Ready Composer? If you search Google for the phrase "World-Ready Composer," the top link is to a Thomas Phinney blog posting that will link you to some free scripts that will turn the WRC on for you. The ones you'd want to use are called "r2l Assign World-Ready Paragraph Composer" - you can ignore the fact that they're all labeled "r2l" as all those two "Assign WRC" scripts do is just that, assign the WRC to either selected text or to a paragraph style.
CS5.5ME + WRC + whatever stuff Winsoft did to make the ME edition = a complete mystery! The ME edition has a bunch of extra Winsoft-specific stuff in it, and I think it quite possible that some code for something Middle-Eastern-specifi or R2L specific is interfering with your Thai. Just a guess, though.
Also: I don't know if this is actually necessary or just magical thinking on my part, but you should mark your text with "[No Language]" if you haven't already done so.