iButton (Score 5, Informative)

Have you considered using DalSemi's iButton (www.ibutton.com)? It's basically a smart card encased in a stainless steel can. Java-based computer, RSA accelerator, etc. Decent documentation, free drivers, and a free IDE are available, supporting Windows and Linux. Programming for them is pretty simple, and they are about as secure as you can get--any attempt to compromise the button zero's the memory. And they are dirt cheap (relatively), the readers are about $15, and crypto buttons start at ~$35. They can also be used to control doors too (check out www.ibuttonlock.com)!