AFAIK polling happens every 30 seconds. If someone submits a post in a topic while you are composing a reply, that other post will be appended to the topic while you are composing your reply.
I’m not so sure a type of “another member is currently replying to this topic” message would be a good thing. I most certainly would not want a “your reply will be put into queue and be posted as soon as it’s your turn” to happen.
We have had a few “sessions” that went a lot better than they could have gone. The discussion flow is a bit disjointed similar to chat but IMHO not that bad.
You can see an example here
http://community.sitepoint.com/t/microsoft-q-a-with-sitepoint-azure-mobile-services/115931