I don't have a service manual for you, however I've taken apart my Powershot S500 several times, including opening the lens/sensor assembly to successfully clean dust on the sensor.
I documented it in this teardown report here on ifixit. From other Powershot cameras I have seen and poked around with, the construction of others is very similar so even if you're wanting to do this with a different model, you may find my photos instructive.
I also notice there are some additional repair guides for other Canon models here on ifixit now - take a look at those also.
if you are handy with taking apart things and handling tiny screws and delicate ribbon cables, you may be able to take apart your camera without a service manual. basically just take it apart one screw at a time, save and label where the screws go, and perhaps take photos along the way (that's how my teardown photos came about). then reverse the process when done. good luck!
If you do get them all apart, I would encourage you to document the process. http://www.ifixit.com/Guide/new
by Alex Jansen