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In Minecraft Vanilla SMP, having access to the server, can I somehow retrieve the position of an offline player?

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Yes. Use NBTedit or its successor, NBTExplorer.

Download and run it, choose FileOpen, go to your Minecraft server directory, find players directory, and then select the name of player you want, for example Player.dat.

You should see something like this:

Player.dat in NBTedit

You need to go to "Pos", and there you have it, coords of player you wanted.
Just double click any to edit it.

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