The H recently wrote about the AlaMode board which lets you use Arudino compatible shields with the Raspberry Pi. The board connects to the GPIO headers of the RPi, and exposes the familiar pin-set of the Arudino Uno on top. The AlaMode board also contains a real time clock, which it can provide as an add-on to the RPi which as none.
In fact, from the picture, it looks like the board is a full fledged Arudino itself, with an Atmel 328P chip on top. It should mean that you can program it on-board, directly from the RPi.
The board is selling for $45, but is out-of-stock at SeeedStudio.