Mac OS X Lion: Virtual Desktops

• add a virtual desktop
• open Mission Control (4 fingers up on touchpad)
• move the mouse to the top right corner of the screen, a + icon will appear there
• click it to add a new desktop
• remove a virtual desktop
• open Mission Control (4 fingers up on touchpad)
• mouse over the desktop you want to delete 
• a small cross(delete) icon will appear in a short while
• click it to delete the desktop

See Also: http://www.tuaw.com/2011/07/20/mac-os-x-lion-and-mission-control/