It should absolutely be off by default. The way these things spread is one tweet that is your message, then the next tweet that says "sorry I didn't mean to post that". Just make a button or a checkbox that says "Tweet this" and lots of people will do it. The behaviour should be opt-in, just like email subscriptions.
100% turn it off. you stand to piss a lot of people off very fast if you dont. other than that, i like the concept. reminds me of www.halfbakery.com but twitter powered. bookmarked it.