I just tried another upgrade and it failed. The log attached to the landscape activity doesn't show any problem, but the /var/log/dist-upgrade/main.log file on the computer nails it down pretty well:
2010-03-12 15:53:57,631 ERROR Not enough free disk space The upgrade is now aborted. The upgrade needs a total of 37.7M free space on disk '/boot'. Please free at least an additional 19.3M of disk space on '/boot'. Empty your trash and remove temporary packages of former installations using 'sudo apt-get clean'. (None)
I'm raising this to Medium because really, without that main.log output, there is no way to even identify the problem using Landscape.