Yes, it happens to me on occasion, but as mentioned it is on the sensor. At first I used a sensor loupe to examine it, then a dry sensor swab to sweep away the stubborn object. Now I just use a dry sensor swab, without using the sensor loupe, just to get those stuborn objects that the "auto cleaning" feature in the camera, my Rocket, or sensor brush won't remove. I change lenses a lot and no matter how careful I am, crap finds its way to the sensor (actually the AA filter on non modified cameras). It doesn't happen often, but it will happen to us all, eventually.
If messing around in your sensor box makes you squeemish, take the advice mentioned in this thread and get Canon to clean it up for you. I know this is an oldish thread and the OPer has probably moved on, so I hope others might find my contribution useful.
-FD
