Edit:
Sorry, I still have this map#0 bug in my 1.4.2 world. But I created now a new 1.4.3 world and I can't reproduce the bug there. So it seems the map#0 in my 1.4.2 savegame is corrupted but for new worlds the bug is already fixed.

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This bug affects only the first map you create in minecraft (map#0). even when you craft a map of the same area as map#0

Reproducing the bug (in single player mode, not testet in SMP):

• craft a map#0 and walk around
• place the map in a item frame at a wall
• craft another map#1
• walk out of the area of map#0
• use map#1
• walk back to the wall an see that map#0 is blank now.

When you reload the world map#0 is visible again. The content of map#0 will disappear again, when you create and use the next map.