Yes but only using a Java enabled web browser with Java 1.3 or higher installed on a Windows computer.
For more information on hosting with Java, please see this article.

Currently participants that use the GatherPlace GP4 software to join a meeting cannot share a single or specific monitor or region of a monitor from a computer that has multiple monitors*.
You can however choose specific applications to share** and move those applications to a single monitor.
Using this method will make it easier for the host to manage the meeting while still having other non-shared applications on their other monitor(s) that are not necessary for the presentation.
See this article for information on Windows application sharing.

* On Macintosh computers only the primary monitor is shared. Therefore, on multi-monitor Macs, it is possible by placing the desired applications/windows on the primary monitor and moving the other windows/applications to the second or third monitor where they would not be part of the presentation.

**Windows computers only. Mac application sharing will be available in the future.