Reuters has published new social media guidelines in its Handbook of Journalism. As well as reminding journalists to keep the personal separate from the professional, it advises:
Can I break news via Twitter?
As with blogging within Reuters News, you should make sure that if you have hard news content that it is broken first via the wire. Don’t scoop the wire. NB this does not apply if you are retweeting; (re-publishing) someone else’s scoop.
If a correction is required, a new tweet that begins “CORRECTION:…” should be published.