Today I released the 2nd beta version of
Teensyduino, my add-on package for Arduino-0012 that allows sketches to be built and run on the Teensy USB board. Version 0.2 fixes many bugs and has more robust coordination between the Arduino IDE and the Teensy Loader.
The main different between the Teensy and a regular Arduino (or the main Arduino clones) is the Teensy has direct full speed USB (12 Mbit/sec) instead of a slow serial port converted to USB by another chip. Teensy can be programmed to act as almost any kind of USB device (including keyboard, mouse, joystick, generic HID, etc), whereas a serial-to-USB board can only ever act as a serial-type USB device.
So far, Teensyduino only adds a serial type device. The Arduino IDE is built with a lot of serial assumption throughout its code. In time I intend to add non-serial options to Teensyduino, though making them work in the same simple way with Arduino-0012 is a massive project. 0.2 is a step in that direction, with major improvements in stability of the code.