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When using the serial field token with the auto node title module, the node must be saved twice to pickup the serial value. Presumably this is because the serial field is the last thing that gets run so the token isn't populated for auto node title on the first save.
Others have tried to do this same thing using NID as discussed and patched here: http://drupal.org/node/194197
Can you have a look at that code to see if it will work with the serial field module like it does with NID?