Dust Devils!

Those dust spots are on your sensor by the way.

the mirror just reflects the image that comes throught the lens around the prism and into your eye. When you take a pic the mirror pops up (and stops reflecting the light, hence the shutter goes black) and allows the light to pass through where the mirror was, through the open shutter curtain and onto the sensor. so any dust on the mirror is not present when the picture is taken....but...dust on the mirror can shift insode the camera and come to rest on the sensor.

When shooting film you effectively get a new sensor (frame) with every shot. With digital its the same sensor all the time. So everytime you change lenses, dust gets inside, sits on the mirror, inside the camera and finds its way to the sensor. Thats what your seeing. Dust sitting on the sensor. RElatively easy and cheap to clean if your confident yourself. OR if not leave it in to a cam shop to get them to clean it.

It's good practice to keep your mirror clean and remove dust. It stops that dust finding its way to the sensor.

Youtube tutorial on sensor cleaning, there are many different tutorials on Youtube
http://www.youtube.com/watch?v=9SCWU03tO1A&feature=related