This is not supported. Users need to be able to access the HOME and BACK buttons when they are not available by the device off-screen.
Some Android devices that are running 2.x will get upgraded to 4.0. My understanding is that those devices will not have the bottom system bar, as those devices will have off-screen HOME and BACK buttons. Whether the top portion of the system bar (time, signal strength, notifications) will be removable via things like Theme.NoTitleBar.Fullscreen
remains to be seen -- we will know more once the Nexus S and similar devices get upgraded.