Normal lenses are not sealed airtight.  When internal elements or groups of elements move, dust or other contaminants can be brought into the lens.  The L series Canon lenses offer greater resistance to moisture and dust through the use of improved seals.  This reduces the likelihood of moisture and dust getting into the lens.  But the seals only reduce, they do not eliminate, the possibility of dust getting inside.

In my experience, the dust is rarely ever a problem, or seen on an image.  Large amounts of dust could cause an overall softening of the image and increase the likelihood of flair.  A single dust particle is of no cause for concern in almost all cases.

Given that dust is such a problem, where is it coming from?  I am assuming that the body itself is sealed so this much be a lens issue.  I can see that my Tamron 24-135, which extends and contracts several inches when focusing and zooming is going to pick up dust.  What about Canon lenses that do not move much, like the 35 F 2?  Are the internal focusing lenses that do not chance size sealed so they would not let dust in?

It seems to depend a lot on F stop and focal length.  I was really surprised when I did some work with my 100 macro at F 22 - I had serious dust issues that I could not even see at F 8!