I made this illustration because it seems so many are without any idea about the probable location of dust in the dSLR...
A: The single-dashed line comes in the lens and bounces off the surface of the reflex mirror (dark blue), becoming the double-dashed green line. Dust or obstructions on the mirror are not in focus in the path of the light and cannot be seen in the viewfinder because they are not focused until they have travelled to the focusing screen. Dust on the mirror cannot appear in the photo because the mirror flips up completely out of the optical path. Use a blower to blow off dust, but normally you do not want to clean off the front surface of the mirror, because the silvering is on the front surface (unlike a mirror at home or in a lady's makeup mirror in her purse).
B: Dust on the lower surface of the focusing screen (green) is in viewfinder focus. Dust on the focusing screen will be seen in the viewfinder but not on the photo because it is not in the optical path to the sensor (orange). It is a nuisance only. If you attempt to clean this, be very careful as it is quite easy to mar the focusing surface of the plastic focusing screen. If you ever try to wash it off, only air dry it rather than blot dry; if you live where there is hard water, rinse in distilled water first before allowing to air dry.
C: Sometimes dust or hairs can get in between the focusing screen and the pentaprism (or pentamirror in the XT/XTi) as shown with the mid-blue line. Dust in between is almost but not quite in viewfinder focus. Dust in between might be seen in the viewfinder as somewhat out of focus, but will not be seen in the photo because it is not in the optical path to the sensor (orange). It is a nuisance only. If you have never removed the focusing screen it is best to leave the cleaning of this area to repair shops. (Only the high end 'pro' camera models have screens that normally are considered user exchangeable, so you would not know how to get in here unless you put in a third-party focusing screen at one time)
D: When the mirror (dark blue) flips up and obscures the viewfinder at the time of exposure, the single-dashed line passes straight and light rays are allowed to strike the sensor (orange). If the dust/obstruction in in the photo and is not seen in the viewfinder, the dust is on the sensor. You can blow the sensor off with an air bulb. As a second-level attempt you might need to use a cleaning solution made for this purpose, using lint-free swabs to somewhat gently swab this. You will not be touching the actual sensor surface, but a clear element in front of it, but that is not a license for using brute force.
Review: A and B and C are nuisances only (although they can drive some people crazy), so you can leave them alone and nothing gets harmed except for the psyche of the compulsively clean. Dust in D is the only situation in which you truly need to take any action.
E. When there is dust on one of the optical surfaces of the lens itself, that dust will not be visible in the photo as localized spot. However, dust in the lens will contribute to inifinitessimal amouts of degradation to the image quality, and lots and lots of dust will soften the image. (Some medium/large format 'soft' portrait lenses of the past had intentionally put a large disk into the optical path specifically to soften the wrinkle lines, and you could dial in the degree of softness!) Great debates about the amount of 'acceptable' dust prevail about some lenses in the Canon lineup. But many lenses in the past -- even ones not intended to be 'soft focus' -- have had bubbles within the lens element itself, which were tens of thousands times larger than any single dust particle (or all of the dust particles combined!), without obvious effect to viewers...I never heard anyone comment about "You can see where there is air bubble in one of the elements and it shows up here in the photo!" So carry the air bubble logic over to dust, and it is debatable if one should fret over small amounts of dust in the lens (That is not to say that you should not blow dust off the front element periodically. I'm merely indicating that dust on the lens is not fatal to your photo!)
