So, apparently MacOS X Lion has been released and it's (currently) only available via an AppStore download which allows for an in-place upgrade from a running MacOS X Snow Leopard system to the newest one. While this may be the way to go in 2011, I still wanted to be able to boot from an optical medium - just in case. Here is how I did it:

Use AppStore to download the Mac OS X Lion installer. Unfortunately on this particular system the AppStore would freeze after clicking on the Buy! button in the AppStore. A workaround suggested to enable Spotlight. Indeed, Spotlight was disabled on this system, because I don't need/use it. But how to enable it again?

  # mdutil -E /Applications
  Spotlight server is disabled.

Hm. Where is that magic .plist file again, to enable Spotlight again again? Ah, here it is:

  # launchctl load -w /System/Library/LaunchDaemons/com.apple.metadata.mds.plist

Now that Spotlight was enabled, I tried again. After clicking the Buy! button again, AppStore would not freeze/hang but instead come up with an error message now:

  Your request is temporarily unable to be processed. Please try again later.

Trying again (later) did not help, searching the interwebs did:

  On the iTunes toolbar click Store and then click Authorize this computer.

After doing that and a bit of fiddling around with iTunes, agreeing to the iTunes Terms & Conditions and restarting AppStore, it would finally start to download the MacOS X Lion installer. But where to? iftop revealed that Port :53218 was busy downloading data via HTTP:

  # lsof -li :53218
  COMMAND  PID  NODE NAME
  storeagen 2126  TCP local:53218->[...].akamaitechnologies.com:http (ESTABLISHED)

  # lsof -ln -p 2126 | sort -nk7 | tail -3
  storeagen 2126 [...] /private/var/db/dyld/dyld_shared_cache_x86_64
  storeagen 2126 [...] ../Library/Application Support/AppStore/444303913/mzm.stuhjljp.pkg
  storeagen 2126 [...] ../Library/Application Support/AppStore/444303913/mzm.stuhjljp.pkg

  # xar -t -f ../mzm.stuhjljp.pkg  | grep dmg
  InstallMacOSX.pkg/InstallESD.dmg

Ah, there it is :-)

After the download has completed, we can just follow the initial instructions and burn the InstallESD.dmg on a DVD via Disk Utility.