Although good DSLRs have sealed bodies and often a sophisticated ultrasonic cleaning mechanism on the sensor, you'll anyway run in dust problem after some time. This time interval could be very long, since dust enters when you change lens, and many people use always the same zoom. But if you have many lenses, and you take many outdoor pictures, dust will inevitably lay down on your camera sensor.
Step 1: the swarm of flies
Effects of this bad attitude of dust are not much noticeable, but when a bigger dust grain will come to keep company to his fellows, you'll notice it and you'll probably want to close aperture to 22 or more and take a shot to the sky. Then you'll become frighteningly aware of the affair magnitude!
I knew dust was becoming a serious matter for my dslr, but I took this shot for curiosity during my last travel on mountains, you can imagine my astonishment when I've seen a swarm of flies around the sky in my picture!