I use VirtualBox with Windows installed under Gnome. In seamless mode there are two taskbars displayed: Gnome taskbar and Windows taskbar.
It works just fine but I've problem with Windows taskbar. There is covered by the Gnome window that has been maximimzed. Unfortunatelly VirtualBox does not solve the problem.
Searching Internet I've found quite two ad-hoc solutions for Windows: Desktop Coral and Hawkeye ShellInit. But nothing for Gnome.
Generally I expected some tool allowing a virtual grid defining that limits the size of windows mizmization. The tools for Winodws just implements this idea.
Note that it is very usefull and not only for guest os taskbar in VirtualBox but also it makes easier window arranging. This turns important especially now in order to take advantage of a big screen.