Raspberry Pi Foundation has released first SD card image for $25 computers that will go on sale on end of this month.
Based on Debian Squeeze (6.0), the image includes LXDE interface, Midori web browser, development tools and example source code for multimedia functions.
You can download the image from here (torrents recommended).
Since rPi boards are not available yet, you can keep the image safe somewhere. If you have got limited edition boards, copy the image to SD card by running the command below.
sudo dd if=debian6-17-02-2012/debian6-17-02-2012.img of=(sd_card_path)
Be careful when specifying SD card path as it can wipe out your drives if wrong path is mentioned.
If you really want to try the image now, run it in QEMU. Detailed instructions can be found here.