Technically, most mobile phones are a Twitter Device. But applications built to integrate with Twitter over our API can also be considered devices. Sure you can send and receive Twitter updates over your phone but you can also do that with Facebook. That makes Facebook a Twitter device. (It does other stuff too.)
In fact, there’s lots of different devices for Twitter. I just took a glance at the Twitter Fan Wiki apps page and counted well over 200 “devices” you can use to interact with Twitter organized by Windows, Mac, Linux, Multi Platform, Second Life, Web Apps, Search Engines, Email, Widgets, Other, and more. Neat!