Here is some info on one explanation behind Orbs......
The most common cause / explanation of the problem is common to a lot of digital cameras with small CCDs, not just the Canon digital cameras. Such digital cameras have a greater depth of field compared to 35mm cameras due to their smaller focal distance. This in even higher degree when the camera is set to wide angle.
Because of the above, small dust particles, droplets of water, etc, drifting in the air close to the camera and not within depth of field for normal 35mm cameras, are within depth of field for digital cameras. Under normal circumstances this is not too big a problem, as these are very small. When flash is used its light illuminates these dust particles, water droplets, and these therefore show up extremely obvious on pictures taken. The problem is worsened for most digital compact cameras because the flash unit is positioned close to the lens barrel. Due to the setup and operation of digital cameras using small CCDs, there is not much that can be done about this phenomenon.
There are some measures which can be taken to prevent this problem from occurring as much as is possible.
1: Switch off flash in environments where there are likely to be small dust particles or water droplets in the air. Instead increase the gain / ISO value for the camera to compensate for this lack of flash. This is only possible with cameras for which the gain / ISO value can be adjusted.
2: Use an external slave flash unit further away from the camera and partially cover the cameras flash. These units are not available from Canon, but there are third party companies making such units. Make sure a slave flash unit is chosen which can be set to synchronize with the second flash as used by Powershot cameras. With Powershot cameras, the first flash is used to measure, the second is the actual flash
I still think they are something else though! lol