Hi Balaji -
Currently Tableau supports consuming OData (a RESTful-like Web Service built with AtomPub) directly.
In the next version (Tableau 8) we can consume data via Google Analytics and BigQuery APIs, and we'll also be able to consume data via SalesForce's web service API.
We currently don't support using the payload from an arbitrary web service as a data source, but we're getting lots of feedback that this would be useful, and we're looking into it.
I hope this helps!