These are called 'fireflies'. They occur when a ray of light randomly reflects into a bright light source, usually the sun. These bright dots also appear in some long exposure digital photography.

There are several settings you can change to reduce fireflies:

* Remove unnecessary glass between the sun and your camera
* Ensure supersampling is set to 3 or 4.
* Set your Rendering mode to Bidirectional with MLT.
* Make sure none of your materials have a color of 1.0, 1.0, 1.0 - all materials should have maximum color values of 0.8, 0.8, 0.8 (no paints in the real world reflect 100% of light)
* Remove the dots using a high pass noise filter in photoshop