add comment boxes below submissions
add a comment box below each response. For example, if I wanted my students to ask questions about a topic and submit them, other students could answer them right under the students question.
Out of scope at this time
Interesting idea. I'm not sure there is a simple 'technological' solution that would allow students to respond to one comment on a particular poll from a cellphone. I feel an in class discussion stimulated by a live poll would still remain a simpler and easier alternative. The "community message board" scenario you present may be best fulfilled by an existing social service such as twitter or Facebook. Here's a great article about the educational benefits of Facebook: http://mindshift.kqed.org/2011/08/50-reasons-to-invite-facebook-into-your-classroom/