I used to think of web service orchestration as (UNIX-like) pipes for the web, where the output of one operation can be fed as the input to the next one.
So I was surprised to stumble across Yahoo Pipes. It's an RSS data (for now) processing and transformation service. It would be more interesting to able to wire together arbitrary web services (they do mention that's coming), but this might be worth checking out.
The other issue with a public mashup/data-flow service accessing public data is that joins, transformations and rendering take precedence over authentication and authorization, both of which are critical first steps in an enterprise application.