Tableau does not have a standalone ETL tool. A number of features in Tableau Desktop have grown from the need to reshape and clean data, including: field aliases, ad-hoc groups, sets, calculated fields and pivoting on measure names / values. We are interested in broader challenges around cleaning, shaping and integrating data but I cannot comment on a timeline or roadmap for additional features or dedicated tools.
I've found Pentaho Kettle does the business for easy enough ETL alongside Tableau - http://kettle.pentaho.com/
Kettle's well worth a look at for dealing with multiple data sources if you need to get them all into a single place for joining when using them in Tableau (as I have). And, as Robert's said, there's enough data-centric features in Tableau you don't have to deal with everything in the ETL. The auto date hierarchies are great.