put a simple e-mail address form on the front page of discourse.org saying “if you want to know when hosted discourse is available enter your e-mail here". Optionally, provide two different lists–one for those interested only in free hosting and one for those prepared to pay.
Totally, @codinghorror is working on something like this, it should be up fairly soon.
It is unlikely we will be offering free hosting any time in the foreseeable future, however we are considering a 1 month free trial (you take your data if unhappy for whatever reason) nothing is set in stone yet.
Yes, good idea, we will definitely be doing this soon. Just a few things to work out first.