Other useful curl commands...
Get a form list
curl --header "X-OpenRosa-Version: 1.0" https://example.appspot.com/formList
Submit a form
curl -v -H "Content-Type: multipart/form-data" -F "xml_submission_file=@/tmp/submission.xml" https://example.appspot.com/submission
Approaches to stress testing ODK servers also has some useful tools if you want to post lots of submissions.