What would it take to expose a Kettle transformation as a (web) service? REST, SOAP, anything will do.
I think you need carte:
http://wiki.pentaho.com/display/EAI/Carte+User+Documentation
Or perhaps just use the BI Server - very easy to setup webservices in there, and being overhauled/made even easier in the next major release.
The enteprise edition has something called a DI server. Which presumably does this. But then you'd know that if you were an EE customer!