Once again, it needs to be addressed that what you say on twitter can be used against you in a courtroom. Hundreds of defamation cases have been filed against individuals and companies for their tweets. There is no legal precedent for defamation cases involving twitter which makes it extremely difficult for a judge. This is why most cases are settled out of court.
There have been many high profile defamation cases on twitter. Recently, an AP reporter was sued for his comments about a few players during a basketball game. Another more notable case was Courtney Love, who was forced to pay a substantial amount after her rants on twitter.
Just remember this, just because you do not mean harm or malice, does not mean that you cannot be sued. Don’t say anything on twitter that could damage someone’s reputation. Twitter is meant to be a fun way to communicate with people, not a breeding ground of rants and harmful comments.