I wasn't at all satisfied with the results of my prior sensor cleaning and was noticing some significant spotting, particularly with some flash shots on my 28-135mm lens. I did some quick tests tonight to confirm the sensor/filter issue and the results confirmed by suspicions.
This was taken with my 30mm:
This with my 28-135mm:
There was a great article in a recent Shutterbug magazine about this very problem and the Dust-Aid product was recommended. It comes in a convenient, easy to carry Band-Aid type metal box and the instructions couldn't have been easier to follow. They even have a disclaimer about Canon filters and say they'll replace it if their product does any damage so I felt confident going into the process. The results were spectacular - I could find no evidence of any dust on the sensor/filter after applying Dust-Aid. The product isn't cheap but I suspect I'll only need to use it every 2 months, so it should last a couple of years. Well worth it so consider it if you are having similar problems.