I have some mystery on my both MacBooks (Air & Pro):
If I used iTunes at least one time during the uptime, the system never shuts down, it just hangs on a blue screen with the spinner rotating forever. If I don't run iTunes, everything is perfect.
The magic is that both MacBooks have different OS: Lion & Snow Leopard, different software installed. The only thing that was migrated from Snow Leopard to Lion is iTunes and my user account.
But creating different user and complete iTunes removal & reinstalling doesn't help.
launchd-shutdown.log seem ok, killing iTunesHelper process before shutdown doesn't help either.