Dust on the sensor is a fact of life with DSLR's. Like someone said on the other board, with film you didn't worry about it because your "imaging sensor" changed everytime you pressed the shutter release. Dust can easily get in if you aren't careful (and even if you are).
Some tips to help keep it dust free: Turn the camera off when you change lenses. Point the camera down (lens mount facing the ground) when the lens is off. Change lenses as quickly as possible. Don't change lenses in a dusty environment (like the beach on a windy day...).
I use one of these to keep the inside of the camera clean: http://www.bhphotovideo.com/bnh/cont...=259157&is=REG
Sensor swabs are a standard way to do it too but I personally haven't done that yet (had mine since August). This website shows more about it: http://www.pbase.com/copperhill/ccd_cleaning I bought the tool he sells, but haven't had the nerve to try it yet. Whatever you do, do not use compressed air on the sensor!