UPDATE: I recommend Raspbian "Wheezy" image as most WiFi drivers and firmware are included
NOTE: for just the r8712u driver: http://www.element14.com/community/servlet/JiveServlet/download/44948-7-97488/r8712u_ko.zip
In the course of testing WiFi adapters, I needed the r8712u kernel module which resides in the staging driver directory of the Linux kernel source in Raspberry Pi's github repo. Fedora Remix and the 2012-04-19 Arch Linux includes staging drivers, but the Debian image does not (as of 2012-04-19 version). Thus, I had to cross-compile on my laptop (x86 Ubuntu Linux 11.10) the Linux kernel including all the kernel modules.
1) The eLinux wiki has a good guide to cross-compiling the ARM Linux kernel for the Raspberry Pi on whatever your computer is:
http://elinux.org/RPi_Kernel_Compilation
2) When you get to the step of:
make ARCH=arm CROSS_COMPILE=/usr/bin/arm-linux-gnueabi- oldconfig
There are couple options:
a) I ran the command above which asks a very large number of questions, one of which is CONFIG_STAGING. You'll need to answer 'y' to that one if you want to build the staging drivers (which was my goal). Also be sure that whatever driver you need is set to either 'y' or 'm'.
b) Another option is to copy /proc/config.gz from the Pi running Debian, gunzip it, and save it into your kernel build directory as .config. Then when you run the above command there will only be a few questions to answer, however you'll need to edit .config to make sure CONFIG
3) Compile the kernel following the next step in the eLinux wiki instructions. It will then take some time to compile the kernel depending on how fast your system is.
4) Now that everything is compiled, we need to be able to create the directory kernel modules that will be copied to the Pi. We don't want the kernel modules to go to the actual location on our system (if running Linux like me), so instead INSTALL_MOD_PATH specifies where they should go:
make ARCH=arm CROSS_COMPILE=/usr/bin/arm-linux-gnueabi- INSTALL_MOD_PATH=/tmp/module modules_install
Replace /tmp/ with wherever you want to store the files which will then be later copied to SD card.
5) Put your SD card with the Debian image in your card reader connected to your computer. Your computer should mount two partitions: the smaller FAT partition is the "boot" partition and the bigger Linux partition is the where these kernel modules will go. You should see "/lib" at the root level of the Linux partition:
a) Backup "/lib/firmware" and "/lib/modules" directories on the SD card (to something like firmware.orig and modules.orig)
b) Copy the "lib/firmware" and "lib/modules" directories from your INSTALL_MOD_PATH to "/lib/" on the SD card
6) That takes care of the kernel modues, but we still need to finish preparing the Linux kernel and copy to the boot partition. The eLinux wiki article descirbes this important step:
http://elinux.org/RPi_Kernel_Compilation#Final_step:_Making_the_.27kernel.img.27_for_your_Pi
NOTE: I was able to load the staging drivers I compiled on the latest Debian (2012-04-19) without having to copy my compiled kernel, too
Finally, I've attached my .config along with /lib/modules and the kernel image that I built on 2012-04-19 from an up-to-date clone of the Linux kernel sources from Raspberry Pi's github. This new kernel and modules are working ok for me with the Debian 2012-04-13 image. If you are reading this not too far in the future, then you can probably just download these files and not bother with compiling.