Note: This article is part of my Testing stuff with QEMU series.
From the Debian GNU/kFreeBSD port page:
Debian GNU/kFreeBSD is a port that consists of GNU userland using the GNU C library on top of FreeBSD's kernel, coupled with the regular Debian package set.
Q: Why would anybody want to do that?
A: Why not? 
So, after we have talked about that, let's start:
apt-get install qemu
Download the latest Debian GNU/kFreeBSD installer ISO image (either for i386 or amd64):
Create a QEMU image which will hold the Debian GNU/kFreeBSD (i386) installation:
qemu-img create -f qcow2 qemu_kfreebsd_i386.img 5G
Boot directly from the ISO image and install Debian into the QEMU image:
qemu -boot d -cdrom debian-20070313-kfreebsd-i386-install.iso -hda qemu_kfreebsd_i386.img
The FreeBSD installer will now start. For more detailed instructions see the Installing Debian GNU/kFreeBSD manual.
First you can choose between an "Express" or "Custom" install (I used "Express").
Next you end up in the partitioning tool. Type "a" to use the entire (QEMU) disk for the installation (the disk is called "ad0", not "hda" as on Linux). Type "q" to quit the partitioning tool.
You are now asked which boot manager to use. For QEMU you should use "BootMgr", the default FreeBSD boot manager. If you install on real hardware you can also use GRUB; in that case choose "None" here (see the manual for more information), but note that the installer does not install or configure GRUB for you! You should do that beforehand!
Next up: The disklabel editor. Here you'll create a partition ("slice" in FreeBSD-speak) for the root filesystem and a swap partition.
Press "c" to create a new slice (will be called "ad0s1"), enter "4GB", choose "FS" (filesystem), and enter "/" for the root filesystem. Per default the UFS2 file system will be used. To create the swap partition, press "c" again, enter "1023MB", and select "swap". The new slice is called "ad0s1b". Press "q" to quit.
Choose "minimal" when asked which distribution to install.
Installation media dialog: select "CD/DVD" and "acd0" (for QEMU's ATAPI/IDE CD-ROM drive).
The installation will now begin, and after a while you're asked to switch to console 3 using
ALT-F3. Do it.
You'll have to answer a bunch of questions: geographic area + city you're in (for timezone), whether you want to participate in the Debian popularity contest, whether module-init-tools should load additional drivers (no, so press ENTER three times). The installation will soon be finished.
At the end you must select "No" as you're told to do, then reboot via "Exit Install". You can then shutdown QEMU.
Restart QEMU with the newly installed Debian GNU/kFreeBSD:
qemu -hda qemu_kfreebsd_i386.img
Press enter at the FreeBSD boot manager prompt, then login as root (there's no password).
First things first: Set up a root password:
Now let's fix networking, update the system and install a bunch of packages:
Yes, there's no vi, not even a symlink to nano! Uncomment the two "ed0" lines ("ed0" is the equivalent to "eth0" on Linux, I guess).
apt-get update && apt-get dist-upgrade
apt-get install vim xorg icewm xterm
You can fix your console keymap using the kbdcontrol package (just select your keymap from the menu):
apt-get install kbdcontrol
Finally, let's fix X11 and start it. But first we create a new user, as we don't want to run X11 as root:
The mouse device is "/dev/psm0", the protocol "PS/2", and the graphics driver should be "vesa":
Option "Device" "/dev/psm0"
Option "Protocol" "PS/2"
That's about it. Login as "uwe" (or whatever your username is) and start X11:
Wasn't all that hard, eh? Now, if you've got some spare time, head over to the Debian GNU/kFreeBSD wiki page and help improving this port ;-) You should probably start with reading the PORTING guide.
Both kfrebsd-i386 and kfreebsd-amd64 seem to be reasonably stable already (and more than 70% of the whole Debian archive builds fine on these architectures, see kfreebsd-i386_stats and kfreebsd-amd64_stats). I'll quite likely install kfreebsd-amd64 on one of my boxes soonish and start using it, maybe I'll even find some time to fix/patch/port some packages...
 More elaborate answer(s) and reasons are available in the Debian wiki.