According to Wiki, it's a dish introduced sometime back in the 19th century and is considered "Royal Thai cuisine" by some. I'm no expert on it so I'm gonna repeat what's already in the Wiki entry. A firmer variety of rice is used instead of the regular Jasmine rice you find in Thailand. The flower-scented water is carefully prepared before the rice is soaked in it. There's quite a spread of specially prepared side dishes to go with the iced rice. Mango, cucumber, fresh spring onions go into the rice which is what I did or I suppose you can also just have them without tossing them in.
When we had it for the first time for lunch in a little neighbourhood restaurant Klang Soi, it was the tempting side dishes that stole the limelight here. I specially like the little shrimp paste balls very much. The shredded beef floss and Chinese radish or chai poh were also excellent. One of the most memorable overseas meals I ever had. Here's another article about it. So the next big question is, can you find this dish anywhere in Singapore?