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If you're data is coming from SQL, you can create a function in the SQL database and then use Tableau's RAW SQL pass-through calculations to call it.
A SQL function is shown here (the one by rajdaksha): http://www.sqlteam.com/forums/topic.asp?TOPIC_ID=135026 seems to work.
It took awhile for me to get back to this, but I finally did and was successful. I wrote a SQL function in Oracle, and then call this function with RAWSQL_REAL and pass it the Standard Deviation that I calculate in Tableau.