I think it's more simple than the suggestions above. If you shoot in the creative modes (Av, Tv, etc) then you'll get a RAW+Jpg (assuming you've configured the body for that option) but any shots taken in the basic modes will only generate Jpg's
Bob
But that's what we were saying
Not quite. On Canon Cameras if you shoot Auto then the camera won't save as RAW even if you have RAW selected. It'll only save as JPG (although I hear some of the very latest models this has changed).
So, if you have RAW selected but are changing between Auto and Manual (or semi-manual) modes then some files will save as JPG and some as RAW.