Because using a Bukkit plugin just to edit a NBT file is ridiculous! Tested on 1.5.x, see /setworldspawn on recent Minecraft releases.

mc_change_spawn.py

#!/usr/bin/env python
#
# This file is placed in the public domain.


import struct


coordinate_name_map = {'x': 'SpawnX', 'X': 'SpawnX', 'SpawnX': 'SpawnX',
                       'y': 'SpawnY', 'Y': 'SpawnY', 'SpawnY': 'SpawnY',
                       'z': 'SpawnZ', 'Z': 'SpawnZ', 'SpawnZ': 'SpawnZ'}


def change_coordinate(nbt, coordinate, value):
    name = coordinate_name_map[coordinate]
    location = get_coordinate(nbt, name)[0]
    assert location
    first_half = nbt[:location]
    second_half = nbt[location+4:]
    return first_half + struct.pack('>i', value) + second_half


def get_coordinate(nbt, coordinate):
    name = coordinate_name_map[coordinate]
    found = nbt.find(name)
    assert found > -1

    location = found + len(name)
    bytes_ = nbt[location:location+4]
    value = struct.unpack('>i', bytes_)[0]
    
    return (location, value)


def get_all_coordinates(nbt):
    return {
            'x': get_coordinate(nbt, 'x')[1],
            'y': get_coordinate(nbt, 'y')[1],
            'z': get_coordinate(nbt, 'z')[1]
            }


def change_all_coordinates(nbt, x, y ,z):
    nbt = change_coordinate(nbt, 'x', x)
    nbt = change_coordinate(nbt, 'y', y)
    nbt = change_coordinate(nbt, 'z', z)
    return nbt


def file_is_gzipped(file_):
    try:
        magic = file_.read(2)
    except AttributeError:
        with open(file_, 'rb') as f:
            magic = f.read(2)
    return magic == '\x1f\x8b'


if __name__ == '__main__':
    import gzip
    import sys

    if len(sys.argv) != 5:
        sys.stderr.write('Usage: {0} level.dat X Y Z\n'.format(sys.argv[0]))
        exit(1)

    level_dat = sys.argv[1]
    x = int(sys.argv[2])
    y = int(sys.argv[3])
    z = int(sys.argv[4])

    if file_is_gzipped(level_dat):
        o = gzip.open
    else:
        o = open
        
    with o(level_dat, 'rb') as f:
        nbt = f.read()

    nbt = change_all_coordinates(nbt, x, y, z)

    with o(level_dat, 'wb') as f:
        f.write(nbt)