Content type via URL parameters?

[J. Matthew Pryor]

In previous discussions I had sen somethign about being able to  
"coerce" particular content type by specifying a parameter in the  
incoming URL, something like http://host.com/resource?media=json

But I can't turn up anything in the documenation. Can someone please  
point me in the right place. Also does this work with the  
ServletConverter style of reslet ?

Thanks,
Matthew