Hi ;
I post here a  small procedure which allows to install a virtual machine ( Debian Lenny with 
User-Mod Linux ) for ispCP testing purposes.
Here, I assume you use Debian (Etch or Lenny).
I. Installing needed packages
Code:
# aptitude update
# aptitude install uml-utilities umlrun user-mode-linux user-mode-linux-doc
 
I.a add your normal user in the uml-net group
Example :
Code:
# adduser nuxwin uml-net
 
II. Creating the workspace
Code:
$ mkdir -p ~/ispcp_uml/tools
 
III. Building the virtual drive
Code:
$ cd ~/ispcp_uml
$ dd if=/dev/zero of=lenny_rootfs bs=1M count=0 seek=3072
 
IV. Download netinstall iso (here, Debian lenny)
Code:
$ wget http://cdimage.debian.org/debian-cd/5.0.2a/i386/iso-cd/debian-502a-i386-netinst.iso
 
V. Downloading and decompressing perl scripts to extract the files in an iso
Code:
$ cd tools
$ wget http://stevieo.freeshell.org/uml/iso-perl-stuff.tar
$ tar -xf iso-perl-stuff.tar
 
VI. Extraction of the Debian installer from the iso
Code:
$ perl -Iiso-perl-stuff iso-perl-stuff/isocat.pl ../debian-502a-i386-netinst.iso \
install.386/initrd.gz > ../uml-initrd.gz
 
VII. Launch of the Debian installer in a UML instance
Code:
$ cd ..
$ linux mem=256M ubd0=lenny_rootfs ubd2r=debian-502a-i386-netinst.iso \
eth0=tuntap,,,192.168.0.110 \
root=/dev/ram rw ramdisk=10240 initrd=uml-initrd.gz \
netcfg/disable_dhcp=true hw-detect/start_pmcia=false debian-installer/probe/usb=false \
debconf/priority=low
 
Please, change the address Ip by your host system IP.
After running this command, the window diologue Debian Installer appears in the current terminal and 3 other virtual consoles are automatically created, two of them being shell.
VIII. Installation
Before doing anything through the Debian installer, you must mount the UML kernel modules as follow:
Select a virtual console (shell) and press enter. Then, run the following command:
Code:
UML# mount -o /usr/lib/uml/modules -t hostfs hostfs /lib/modules
 
This being done, return to the terminal of the Debian installer and follow each step.
1. Choose language :
Code:
English
 other
 Europe
 France
 en US.UTF-8
 select : fr_FR.UTF-8, fr_FR, fr_FR@euro and select continue
 
Note : This is an example but you can override several values ( Europe, France... )
Do not select "Select a keyboard layout" step, jump to the next step.
2. Detect and mount CD-ROM :
Code:
No
 No
 yes
 none
 /dev/ubdc and Continue
 Continue
 
3. Load installer components from CD :
4. Detect network hardware :
5. Configure the network :
Code:
No
 Your virtual machine ip addresse ( ex: 192.168.0.130 ) and Continue
 Your netmask ( ex: 255.255.255.0 ) and Continue
 Your gateway (Here, must be the address Ip of your host system - ex 192.168.0.110) and Continue
 Name Server addresses (you dns server(s) address(es) - see your host system /etc/resolv.conf file for this ) and Continue
 Yes
 Your Hostname (ex: ispcp) and Continue
 Your domain name (ex: nuxwin.com) and Continue
 
6. Configure the clock :
Code:
Yes
 Continue
 Continue
 
7. Detect disks :
8. Partition disks
Code:
Guided - use entire disk
 /dev/ubda - 3.2 GB User-Mode Linux UBD
 All files in one partition (recommended for new users)
 Finish partitionning and write change to disk
 Yes
 
9. Install the base system
10. Setup users and passwords
Code:
Yes
 Yes
 Your user root password and Continue
 Your user root password confirmation and Continue
 Yes
 Your normal user full name and Continue
 Your normal user login name and Continue
 Your normal user password and Continue
 Your normal user password confirmation and Continue
 
11. Configure the package manager :
Code:
yes
 http
 France
 ftp.fr.debian.org
 Continue
 Yes
 Continues
 
Note: This is an example but you can override it with your personnal value (France, 
http://ftp.fr.debian.org)
12. Select and install software :
After this, jump to step "Continue without boot loader"
13. Continue without boot loader
14. Finish the installation
Finally, after reboot, type "halt" to halt the UML instance.
When this is done, your virtual machine is almost ready to be used.
IV. Import UML Kernel Modules
Start your virtual machine by running the following command::
Code:
$ linux mem=512M ubd0=lenny_rootfs eth0=tuntap,,,192.168.0.110 root=/dev/ubda1 ro
 
When this is done, it'll just log into a virtual consoles prompt.
After login, execute the following commands to import any UML kernel modules :
Code:
# mkdir /mnt/modules
# mount -o /usr/lib/uml/modules -t hostfs hostfs /mnt/modules
# cd /lib/modules
# cp -Rf /mnt/modules/* .
# umount /mnt/modules && rm -r /mnt/modules
 
Finally, type 
reboot to restart your virtual machine.
Your virtual machine is ready to use.
Note: It is also possible to use 
debootstrap instead of the debian installer and the netinstall iso image but my method has the advantage of not having to manually configure the virtual machine (swap, hostname, fstab, network ...).
Thank you for your feedback.
References :
http://stevieo.freeshell.org/uml/adminless-uml-d-i.html
http://www.debian.org/releases/stable/i3...03.html.en
Todo: How to compile the UML kernel with Debian. 
Todo: How to applying skas3 patch to improve performance.