Here are the EPA guidelines for duct cleaning and when it is necessary. http://www.epa.gov/iaq/pubs/airduct.html
Keep in mind that the airflow ducts usually aren't as bad as commercials would have you believe, unless the system was run without an air filter. The filter usually sits in your air return, which makes sure any air flowing into your supply ducts is "clean".
So, if you see a lot of crap in your supply ducts, you probably have unfiltered air entering the system (hole in a duct maybe). The other way stuff gets into the supply ducts is from people dropping crap into the floor registers. If it's dirt in the registers, you can usually get it out by lifting the register and sticking your vacuum cleaner's hose in there. Just be careful that you don't gouge or hit the sides of the ducts, which can damage them (they can shed fiberglass).
You may have stuff in your return ducts (animal dander, etc), but because of the air filter, this usually doesn't affect the overall air quality in the house much. The exception would be if you have something in the ducts that the air filter won't trap (mold spores, smelly things).