Recentely Raspberry Pi team release the Debian image for Raspberry Pi (used on alpha board). This image is based on Debian Squeeze (6.0) and comes with LXDE user interface and Midori browser, development tools, and sample code for accessing the multimedia functionality on the device.
Here how to put it in our SD Card:
Download image from Raspberry Pi site via torrent or direct http link
Download disk image tool here or here (official site)
Unzip diskimager & Debian image then run Win32DiskImager.exe
75Mb: FAT 32 Partition with the GPU binary blob and relevant libraries, the kernel image and some configuration files
1,55Gb: EXT-4 Partition with the Debian rootfs
191Mb: Swap Partition
Other 5,64Gb of unallocated space is the remaining space available on our SD Card. We will extend the EXT-4 partition the first time we boot Debian on Raspberry Pi...
UPDATE: added link to official site of Win32DiskImager (https://launchpad.net/win32-image-writer)