It's funny, what with it being Halloween weekend and everything, too... But I've at least 2 conversations about orbs this week, as well as hearing something about them on the radio.
The basic gist of it is that you're using a digital camera that pics up infinitely more details than film cameras ever did, so when there's any refraction of light, it's picking it up, be it dust, moisture, or in some cases, additional lights from another angle, it's going to show.
the last option, more light is what was happening to me at work... I work for Olan Mills, and my photographer had been getting orbs lately, the culprit, he was getting lazy in where he was moving his lights when changing poses! The flash and reflections were just right with the camera to get the orbs.