The following is the current Solr 3.1 format for sending multiple documents by JSON. It's not analogous to the XML method, and isn't easily generated and serialized from a hash in Perl, Python, Ruby, et al to JSON, because it has duplicate keys for "add".
It's cited at this page: http://wiki.apache.org/solr/UpdateJSON
Near the text: "Here's a simple example of adding more than one document at once:"
Here's a better format that's analogous to the XML method of submission, and is easily serialized from a hash to JSON:
The original XML method: