When using your NSLU2 with an alternate firmware, you may encounter problems. I'm facing the problem, that my Slug won't boot anymore and I don't have a RS-232 converter at the moment to connect to my Slug via serial port.
One solution I came to is booting your slugs OS through qemu. I used a 4GB USB-Stick to store my Debian installation.

http://www.aurel32.net/info/debian_arm_qemu.php is a very good resource, I'm using the kernel and initrd images from this site.

This section is under construction.

Assuming your USB-Stick's device node is /dev/sda:

dd if=/dev/sda of=sda bs=512

Also make a backup, we will overwrite the current content of the USB-Stick later. You also could store the partition table, if you are planning to change your image without qemu.

/sbin/cfdisk -P t /dev/sda >partitiontable.txt

The output cat partitiontable.txt

Partitionstabelle von /dev/sda

         ---Anfangs----      -----End------   Anfangs-   Anzahl der
 # Flags Kopf Sekt Zyl.  ID  Kopf Sekt Zyl    Sektor     Sektoren
-- ----- ---- ---- ---- ---- ---- ---- ---- ----------- -----------
 1  0x00    1    1    0 0x83  254   63  479          63     7711137
 2  0x00    0    1  480 0x82  254   63  497     7711200      289170
 3  0x00    0    0    0 0x00    0    0    0           0           0
 4  0x00    0    0    0 0x00    0    0    0           0           0

We need to disable the LED activation scripts, otherwise the system won't boot completely.

qemu-system-arm -M versatilepb -m 256 -hda sda \
-kernel vmlinuz-2.6.18-5-versatile -initrd initrd.img-2.6.18-5-versatile \
-append "root=/dev/sda1 single"

a more elegant way is following way to run your configuration

qemu-system-arm -M versatilepb -m 256 -hda sda -nographic \
-kernel vmlinuz-2.6.18-5-versatile \
-initrd initrd.img-2.6.18-5-versatile \
-append "root=/dev/sda1 console=ttyAMA0 single"

This will use your terminal window for output, this should be more comfortable than a 80×25 wide display.
This will boot your actual DebianSlug installation in single user-mode.

After a while he finished the boot process and asks for your root password.

During boot you may noticed a message, that eth0 has been renamed to eth2. To bring up your network, just execute:

dhclient eth2

To copy all your changes back to the USB-Stick, you could do a

dd if=sda of=/dev/sda bs=512

Before running this command please ensure, that this device is your USB-Stick. Overwriting a wrong device damages your partition table and could result in data-loss!

During working with qemu and my Slug image I encountered following problems

Turn it on by running

leds disk-2 on

to turn it of again

leds disk-2 off

You can mount the image by using a loop-device. There is a little need of a calculation, so if you have dumped the partitiontable to partitiontable.txt, look up the Start-Sector of the partition you want to mount.
Partition one starts at sector 63 and the blocksize is 512 in this example. The offset will be at:

63 * 512 = 32256

Mounting the loop device:

losetup /dev/loop/0 -o 32256 sda
mount /dev/loop/0 /mnt/misc/

Umounting the loop device:

umount /mnt/misc/
losetup -d /dev/loop/0

set your time be using date or run a ntp client. I have ntpd installed, so executing

ntpd -s

should fix your time.

And before someone tries to blame me for any damage or data-loss. I don't take any responsibility for any mistakes you make.