Submit webform via POST on restful API
https://stackoverflow.com/users/763010/tyler-frankenstein created a grate article that can be found here

you might also find https://stackoverflow.com/questions/10428014/steps-to-make-a-mobile-application-using-drupal-as-a-back-end-or-service/13662041#13662041 useful it has links to articles that describe services in full.

I'd also advise a couple plugins
Firefox: poster
Chrome: postman

http://drupal.org/node/1795770 might be more applicable it's how to login, post a comment, and loggout from curl hope it helps.