Arduino.cc just announced two new Arduino boards: The Arduino Uno, replacing the Duemilanove, and Arduino Mega 2560, succeeding the Mega. The Uno gets a programmable USB chip which can "make the board show up as a variety of USB devices (Keyboards, Mice, Joysticks, MIDI etc.)". The Mega 2560 gets double the memory (256 kB). Both boards keep their respective form factors, so all shields will still fit. There is also rumours of a board with integrated Ethernet.

The new boards are already available from SparkFun at the same price as the boards they replace:

Arduino Uno - $30
Arduino Mega 2560 - $65

The old ones are sold at a discount while stocks last:

Arduino Mega - $50
Arduino Duemilanove - $25

Can't decide which Arduino is right for you? Arduino buying guide!