Today I noticed some blotchiness in my digital photos. This is caused by dust on the CMOS sensor. I came across two interesting articles on how to deal with this problem (without paying a fortune getting this done professionally).
http://www.bobatkins.com/photography/tutorials/sensorclean.html http://www.bobatkins.com/photography/digital/sensor_cleaning_ii.html
I followed the technique in his second article - I purchased a pack of cheap acrylic makeup brushes and lightly brushed the dust off the sensor. Seems to have removed the large particles quite well.
To test the before and after cleaning simply set your lens to manual focus, and focus to infinity. Overexpose your photo by 1 stop and set aperture to the smallest it can go (highest number) Point the camera at a white wall (I also used a flash) and take a photo.
This should give you an idea of how grotty your sensor is.