Thank you! I hadn't thought of the wear-and-tear problem.
There are inexpensive and durable solutions designed exactly for your purpose. Google iButton
I still think the cheaper and more robust system would be to use RFID tags/cards. The link I posted earlier was to give you an idea how they work but if the price is wrong then ebay sell readers and tags/cards for a lot less money. You can either use cards or tags (or mix them up) depending on what suits best.Reader10x Cards10x Tags
I agree with you, but the thing about the RFID cards is that I think they require some special hardware and software to program (or to match with an identity in a software system). I want to create a product that's as simple and easy to set up as possible, for a company with no skilled IT staff. I figured that saving a text file on a USB stick would be hard to screw up. I will try both.