In addition to warren's concerns about guest queueing, at what point in the proceedings will you be copying photos?
People will be snapping photos every few minutes, so unless you want to do a full copy just before each camera-weilding guest leaves you may need to keep a record of what was on the card previously and what is on it now, then doing a copy of anything new.
This could solve the problem of copying old content if guests check-in with your system as they arrive. During check-in just take a file listing, then the guests can update whenever they feel like it, and you won't need to worry about copying their nude / holiday / trainspotting photos.
This approach could introduce the problem of too many visits, but would establish an 'almost-live' sort of feel if you're going with the one-the-day slideshow idea.
On the whole, I'm not sure if the software exists to manage the process fromm start to finish - but it sounds like a good idea to write it.