Blog post introducing Slack-Twitter, Node.js middle for reading your timeline and posting to Twitter via Slack.
Posting message from a Slack channel directly to Twitter.
Character count: the Node.js middleware rejects the tweet if it's longer than 140 characters, as calculated using Twitter's own algorithm, and sends a message to the channel saying so.
Smart quotes (not optional).
RTs and replies are not implemented. You'll have to click the link if it's desktop (which will go to the Twitter website) and I have it set to open the tweet in Tweetbot in the iOS app. (You can choose which app to open Twitter URLs within the Slack for iOS app settings.)
Working, but not implemented in the public release:
One channel, #sports, tweets using a different 'app' (from dev.twitter's perspective), so if you want to mute that app, you can do so.