The Arduino Team recently posted a video of Massimo Banzi introducing the new Arduino Leonardo, which is now available for sale. The Leonardo uses an ATmega32u4 microcontroller, which runs your sketches and also communicates with your computer directly via serial. (Whereas previous boards used a separate chip to handle the serial communication.) This change allows the Arduino to behave as a mouse or keyboard and makes interfacing with computer programs much easier. The new board also has a few more improvements including more analog inputs, one additional PWM output, and an IOREF pin so that expansion shields can determine what voltage level the board is operating on. This last feature becomes important when the Arduino Due comes out, since it will run at 3.3 volts. Those of you who really want to dig into the nitty gritty, check out the official Arduino Leonardo page, which outlines all the differences in this new and less expensive board.
Massimo Introduces Arduino Leonardo