I recently visited Thailand and got lots of in-depth information about it.
It REALLY depends on what you're doing, when you're doing it, where you're doing it, how long you're doing it there, how well-protected you plan to be, and how much risk you're willing to take.
Malaria is more of a problem in northern Thailand. There are more mosquitos during the rainy season. There are lots more mosquitos in the jungle than in the city. If you camp in the jungle, make sure you use mosquito netting while you sleep. During the day, use bug spray on your clothes, especially near your neck, ankles and wrists.
Quoting from elsewhere on the CDC site:
When traveling in Thailand, you should avoid mosquito bites to prevent malaria. You may need to take prescription medicine before, during, and after your trip to prevent malaria, depending on your travel plans, such as where you are going, when you are traveling, and if you are spending a lot of time outdoors or sleeping outside. Talk to your doctor about how you can prevent malaria while traveling. For more information on malaria in Thailand, see malaria in Thailand.
If you are traveling in the jungle for more than a day or two—especially right near the rainy season, I would absolutely take malaria medicine. Better safe than sorry.
Also, be aware of the other infections diseases in the area, such as Japanese Encephalitis, Hepatitis A & B, Yellow Fever and others.