That is just on the list of things people have requested and it's been on the X11 FAQ from Apple since the first beta was released. That does not mean it will be implemented. The X11 folks at Apple are working with the CodeTek people to get Apple's X11 server to work properly with CodeTek's VirtualDesktops. I wouldn't put too much hope in virtual desktops being in 10.3. People thought it would be in 10.0, then said it'd be in 10.1, then said it'd be in 10.2. Until it's in a feature list given to the public, I won't believe it's going to be implemented by Apple. It might be implemented in X11's quartz-wm, but it would only work for X11 windows in quartz-wm. You can already use other X11 window managers and virtual desktops in X11 on OS X as virutal desktops is handled by the window manager in X11 and not by the X11 server itself.
Brian