The lastest 505 to come out, the Fujitsu F505i has a thumbprint sensor. They advertise it as a security device. Once you've registered your thumbprint than you can press your finger to it to unlock your phone. Of course in the fine print it says "This function doesn't always work. When it doesn't work you'll have to unlock it the normal way and use your secret code"

They also claim some games use it including one virutal pet game that comes with it, "Chibiwanfurenzu" (puppy friends), in which case you use the pad to virtually pet your virtual puppy