Copying the image files back onto the card while in the camera used to work on a Fuji finepix i used have.
It would work but if the images where a higher resolution than the cameras best setting they would not playback on the camera itself.
I used to do this to get pictures printed before i got a memory stick and then a card reader and these have so many advantages and are quite cheap now i would recomend getting one.
I gave up trying it through my 400d, but I tried with a Powershot A80 and using Zoombrowser EX i can do it. It even lets me change the quality of the images. Excellent.
I will probably pick up a card reader though as I’d like to do it with my 400d.
I cant recomend getting a reader enough as it saves your camera batteries and the socket on the camera wont get a battering and i think the card readers are much faster then the camera is if you know what i mean :)
Also you dont need any software to transfer images its just a usb device.