The Gertboard is coming - a capable, inexpensive expansion board for the soon-to-ship Raspberry Pi.
The Gertboard is will make it much simpler to interface the Raspberry Pi to the outside world. It provides access to the Pi's GPIO pins, and can include a motor driver.
Gert van Loo of Fen Logic Ltd, the board's designer, will publish all the design documents once the design is stable. Raspberry Pi expect to sell a bare board in their shop. That means that you'll need to buy the components yourself, program the on-board PIC if required, and solder the components.
Since many of the components are SMT (surface mount) you will need to be a proficient constructor to assemble your own board. Gert hopes that someone will start making per-build boards. I suspect there will be quite a market once the Pi starts to shop in volume next year.