If an upgrade is started using the stsadm command line tool, you will not able to start another upgrade process from the user interface. If you try to start a second instance of upgrade, the following error message will be displayed:
"Unable to upgrade SharePoint Products and Technologies because an upgrade is already in progress. Upgrade progress can be monitored using the Central Administration Web Application or by monitoring the upgrade log on server "X"."
"X" is the name of the server that initiated the upgrade process.
stsadm -o setproperty -pn command-line-upgrade-running -pv No