you may not know that your favorite browser comes with a great took to check for dust on your sensor...
in the address bar, type "about:blank" and hit enter. now you should get a blank white browser page... just full screen that baby, put your favorite lens on the camera and take an out of focus shot of the screen with just enough exposure to get a nice white blur... now load the image in an editor with a curves adjustment.... look at the histogram and adjust the curve to maximize the contrast on the image.... now you should be able to easily see the dark spots where you have dust on the sensor.
depending on your camera firmware and editing software, you may be able to convert this image into a dust mask to automatically correct for this dust... if you can do this in the SLR's internal software, you should be able to have it applied to every shot by default.
With my Nikon D40, I don't even need a PC, there's a dust ref menu option that allows you to take a photo which then gets dissected by the camera software to create the dust mask. all of this happens automatically. you just need a good bright white light source like an lcd monitor with a blank browser page. the camera moves the focus to infinity for the shot - this blurs out the tiny black lines between the lcd pixels so they don't interfere with the sensor dust image. note that using this method can potentially cause some weird side effects like strange exposure gradient shapes and such - ghost shapes maybe.
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