MySQL - MySqlDump uses \n instead of null
There are no options for handling the output of NULLs.
You may want to experiment with --hex-blob. This will dump BINARY, VARBINARY, BLOB fields in hexadecimal format. This should make the data portable. You can see what this option produces for NULL values.
mysqldump --user=root --port=3306 --password=topsecret --hex-blob some_database --tab=C:\ARCHIVE\some_database --triggers --routines --fields-enclosed-by=\&quot;&quot; --fields-terminated-by=, --lines-terminated-by=\r\n

Give it a Try and let us all know if this helps.
UPDATE 2012-01-13 18:26 EDT
This may sound very gross but you can pipe the output of mysqldump into sed as follows:
mysqldump --user=root --port=3306 --password=topsecret --hex-blob some_database --tab=C:\ARCHIVE\some_database --triggers --routines --fields-enclosed-by=\&quot;&quot; --fields-terminated-by=, --lines-terminated-by=\r\n | sed 's/\\N/NULL/g'

Of course, you cannot import that into MySQL until you want the string NULL to be the value to be imported.