I also saw this error message during my Hardy -> Lucid server upgrade (which I was doing with a "do-release-upgrade -d" run from the command line).  However, at least in my case the upgrade process continued processing packages after the error (proceeding to configure ones that were already "pending" at the time of the error), and I see that a later attempt to rebuilt the initrd image apparently succeeded.  

Here are the lines from the "do-release-upgrade" output that seem to be related to this issue:

Selecting previously deselected package initramfs-tools-bin.
Unpacking initramfs-tools-bin (from .../initramfs-tools-bin_0.92bubuntu67_amd64.deb) ...
[...]
Preparing to replace busybox-initramfs 1:1.1.3-5ubuntu12 (using .../busybox-initramfs_1%3a1.13.3-1ubuntu9_amd64.deb) ...
Unpacking replacement busybox-initramfs ...
[...]
Preparing to replace initramfs-tools 0.85eubuntu39.3 (using .../initramfs-tools_0.92bubuntu67_all.deb) ...
Unpacking replacement initramfs-tools ...
[...]
Setting up initramfs-tools-bin (0.92bubuntu67) ...
[...]
Setting up busybox-initramfs (1:1.13.3-1ubuntu9) ...
[...]
Setting up initramfs-tools (0.92bubuntu67) ...
Installing new version of config file /etc/initramfs-tools/initramfs.conf ...
Installing new version of config file /etc/initramfs-tools/update-initramfs.conf ...
update-initramfs: deferring update (trigger activated)
[...]
Processing triggers for initramfs-tools ...
update-initramfs: Generating /boot/initrd.img-2.6.24-23-server
[ generation apparently succeeded ...]
[...]
Preparing to replace module-init-tools 3.3-pre11-4ubuntu5.8.04.1 (using .../module-init-tools_3.11.1-1_amd64.deb) ...
Unpacking replacement module-init-tools ...
dpkg: warning: unable to delete old directory '/etc/modprobe.d/arch': Directory not empty
[...]
update-initramfs: Generating /boot/initrd.img-2.6.24-23-server
cpio: ./etc/modprobe.d/arch-aliases: Cannot stat: No such file or directory
update-initramfs: failed for /boot/initrd.img-2.6.24-23-server
dpkg: subprocess installed post-installation script returned error exit status 1
Exception during pm.DoInstall():  E:Sub-process /usr/bin/dpkg returned an error code (2)

Could not install the upgrades 

The upgrade is now aborted. Your system could be in an unusable 
state. A recovery will run now (dpkg --configure -a). 

Please report this bug against the 'update-manager' package and 
include the files in /var/log/dist-upgrade/ in the bug report. 
installArchives() failed 

[... the system continued with more "Setting up...." lines ...]

Setting up module-init-tools (3.11.1-1) ...
Removing obsolete conffile /etc/modprobe.d/aliases
Removing obsolete conffile /etc/modprobe.d/isapnp
Removing obsolete conffile /etc/modprobe.d/options
Removing obsolete conffile /etc/modprobe.d/arch/x86_64
Running depmod for 2.6.24-19-server...
Running depmod for 2.6.24-21-server...
Running depmod for 2.6.24-23-server...
[...]
Setting up initramfs-tools (0.92bubuntu67) ...
update-initramfs: deferring update (trigger activated)
[...]
Processing triggers for initramfs-tools ...
update-initramfs: Generating /boot/initrd.img-2.6.24-23-server
[ generation apparently succeeded ...]


After the do-release-upgrade processing finished, there /etc/modprobe.d/ did not contain anything named *arch*: 
   # ls -l /etc/modprobe.d/*arch*
   ls: cannot access /etc/modprobe.d/*arch*: No such file or directory

It appears that the initrd.img-2.6.24-23-server file was successfully built:
  # ls -l /boot/initrd.img-2.6.24-23*
  -rw-r--r-- 1 root root 8271164 2010-03-09 14:12 /boot/initrd.img-2.6.24-23-server
  -rw-r--r-- 1 root root 8148730 2009-03-10 10:06 /boot/initrd.img-2.6.24-23-server.bak

(However, I haven't actually tried rebooting using that initrd image, since I plan to upgrade to the Lucid version of the kernel first...)