Interesting device I played with yesterday - Tablet that docks into notebook keyboard w/touchpad
The Asus Transformer. I used it for a few minutes and found it to be easy to type on and to navigate with the touchpad. And the touchpad also supports gestures so you can fully navigate the Android interface. The only thing I noticed is that is was easy to tap the touchpad by accident while typing and mess up where you were. There might be a touchpad setting to address this somewhere but I couldn't find it.
This differs from the external keyboards available for most tablets because of the integrated pointing device, USB ports, and card reader that are built in. Oh, and I don't think it uses Bluetooth since the tablet plugs directly into the keyboard.