Arduino2012,
Large warehouses use barcode scanners to keep track of inventory, most things already have a barcode on them.
RFI scanners at each doorway with chips in/on each item would be an elegant if expensive solution.
Either connected, possibly wirelessly thru the Arduino/XBee, to a central computer.
As far as implementation, the object of the professor's exercise was to make you think, maybe we have done enough for you.