Easy Mark, just get her to try out the new camera, pointing out the new features as she gets herself familiarized with the new toy (get the store to whack on a premium piece of glass before hand).
When it's all done, get her to hand back the camera.
Then go home and don't mention it for a couple of days. On day three or so, get her to take some photos with the 400d, but still say nothing.
On day six or seven, casually leave some catelogues from Teds or JB Hi Fi on the kitchen table. When she takes interest in the camera section, casually ask "so what do u think of that canon you tried out the other day?"
Well, you can always try
Bo