OK, so I did lots of digging and found nothing, so I decided to do a rather unscientific experiment and shot an image with the camera set to RAW+JPEG Fine, sending RAW to one card and JPEG to another. I then took the card with the jpeg image and attempted to edit it, creating a negative of the original image, saving it over the original and then putting the card back in the camera. My expectation was that if the camera showed the altered image then we'd know it was the actual image and if not then it would be a stored preview. What I got was a message saying that the file was invalid, so obviously the camera is looking to read the original file.
I then did consecutive shots varying the size and resolution of the JPEG file...
A) JPEG Fine + Large
B) JPEG Normal + Medium
C) JPEG Basic + Small
My expectation is that if the camera is reading the file then it should take longer to read A than B, and longer to read B than C. What I found is that it took just as long to load zooms of A and C, while B loaded very quickly. Figure that one out!!
Anyway, I'm more inclined to believe that when reviewing a JPEG that you're actually getting the full image, while with the RAW you will always get the preview image stored within the file.