Generally, aircraft anti-ice (note anti-ice, not deice) systems only prevent the ice from forming, they can't remove ice that has already formed, although see the post below for B747 specific operations.
These systems (on jet aircraft) use hot air bled from the engine compressors and feed it through ducts in the leading edges of the wings and engine nacelles. This is becaues the ice tends to start to form on the leading edges and work its way aft, so by heating the leading edges it prevents the initial formation. If the wing is already covered in ice, this leading edge only system won't affect the rest of the wings surface, so the ice needs to be removed before flight. Once cleared and the aircraft is flying, the antiice system prevents new ice forming.
Edited in view of new information posted in the reply below....
Last edited by ANOTHER ton?; 21st Mar 2006 at 21:57.