The warning of a page currently being edited by a user in Confluence does not appear in a clustered configuration. The only warning that is every displayed is when the user who saves last is given a warning telling them they are editing an outdated version of the page.

"You were editing an outdated version of this page. Another user has updated this page before you. Your changes against the now updated version is shown below. Please decide whether your changes should overwrite it or you wish to discard your changes."

This was tested on two clustered nodes - one running XP and one on Linux Ubuntu 6.06.
I also replicated this problem with two nodes on the same machine.

Perhaps there is a problem with the Heartbeat not being sent out in a clustered Environment?