OK, I finally got round to replicating this. Under VMware server, I made two separate installations: ubuntu-6.06.1-desktop-i386 and ubuntu-8.04-desktop-i386


(1) On the 8.04 VM, I let it upgrade all packages (203 of them); shutdown; snapshot; restart.

Then I tried "apt-get install ubuntu-xen-desktop". This failed as follows:

The following packages have unmet dependencies.
   ubuntu-xen-desktop: Depends xenman but it is not installable
E: Broken packages

However, "apt-get install ubuntu-xen-server" was successful, and installed 20 new packages.


(2) On the 6.06.1 VM, I let it upgrade all packages (206); reboot; sudo update-manager -d (to upgrade to 8.04); shutdown; snapshot; restart.

After this, trying to apt-get install ubuntu-xen-desktop or
ubuntu-xen-server just said that both packages were unavailable, e.g.

E: Couldn't find package ubuntu-xen-server

Looking in /etc/apt/sources.list, 'universe' was not enabled (and there were
obsolete entries for dapper, albeit commented out). So I used 'synaptic' to
enable the universe set.

After this, apt-get install gave the same errors as I'd originally reported:

# apt-get install ubuntu-xen-desktop
...
The following packages have unmet dependencies.
  ubuntu-xen-desktop: Depends: linux-xen but it is not going to be installed
                      Depends: xenman but it is not installable
E: Broken packages

# apt-get install ubuntu-xen-server
...
The following packages have unmet dependencies.
  ubuntu-xen-server: Depends: linux-xen but it is not going to be installed
E: Broken packages

This is good - it means the problem is repeatable!


(3) Reverting to the snapshots, I installed copied off 'dpkg -l' and /etc/apt from both.

Comparing /etc/apt showed that the fresh 8.04 machine had both 'universe' and 'multiverse' repositories enabled by default, whereas the 6.06.1 upgrade didn't.

As for the dpkg output, the 6.06 box had a lot of old packages in ^rc state, so I grepped these out. But this still left a lot of differences: what I get from
diff -ubB 606 804 | grep '^[-+]'
is attached as dapper-hardy-pkg-diff.txt

I note that the fresh 8.04 install has installed a 'generic' kernel and
modules, whereas the 6.06.1 upgraded machine has a '386' kernel.

(Aside: a fresh 8.04 install seems to pull in a lot less of the kitchen sink
by default, which I think is a Good Thing. A post-update script to remove
all the spurious packages would be useful. Strangely there are also 3
packages which a fresh install gets that an upgrade doesn't)


(4) I tried installing the linux-generic kernel but this didn't make any
difference.

(Aside: updating the kernel also gave me a reminder to read
/usr/share/doc/grub/NEWS.Debian.gz and follow its instructions, which I
otherwise wouldn't have known was needed)


(5) So then I reverted to snapshot and used synaptic to enable both
'universe' and 'multiverse' repositories.

At this point the problem was fixed: ubuntu-xen-desktop was broken as above,
but ubuntu-xen-server installed successfully.


(6) So the conclusions I can draw:
* The ubuntu-xen-desktop metapackage is broken
* The ubuntu-xen-server metapackage works on a fresh 8.04 install, but not a 6.06.1->8.04 upgraded machine
* This is because a fresh install enables both 'universe' and 'multiverse' hardy repositories, but an updated machine doesn't get either by default.
* The fix is to enable these repositories manually

If there's any other useful comparison I can make between these two snapshots, please let me know.

P.S. If anyone wants to duplicate this work, beware that 4GB of virtual disk is not quite enough for a 6.06.1 install followed by an 8.04 upgrade. I redid it with 6GB and it was fine.