I've encountered a map bug in 1.6.4 SMP, which is similar to this one explained in the wiki:
Currently, placing a map on a wall in SMP using an Item Frame possibly causes an issue with maps held by players in which the location marker for the players 'disappear' under explored regions of the map, and preventing unexplored regions from becoming 'explored', effectively making all maps on the server useless. Deleting all of the map.dat files in the world data folder, as well as the idcount.dat file resolves the issue, unless players have maps in their inventory.
The difference is that despite not showing my marker, it does explore new areas, contrary to what is written there.
Also, "Deleting all of the map.dat files in the world data folder, as well as the idcount.dat file resolves the issue" is kind of vague: it may solve the issue, but does it cause any problem? Or are these only cache-like files?