Here are five examples that show how Twitter’s unique platform is creating a new set of media rules that are forcing the law to play catch up:

The Courtney Love Case: Twitter and Defamation

It’s never a good idea to falsely say someone is a drug-addled prostitute who lost their child. But bad-girl singer Courtney Love did just this in a Twitter rage, leading the target of the rant to turn around and sue her.

The case settled last year but it has since raised question about whether normal rules of defamation should apply to Twitter. Some say that tweets are more like an op-ed page than facts, and that they should be considered as mere opinion (which is a defense to defamation).

As one scholar has noted, it can be hard to tell from a 140-character message what parts are fact and what parts opinion. Twitter also raises questions about how to measure the harm caused by a tweet. Should the penalty be the same for Kim Kardashian who has 13 million followers as it is for a village gossip with five followers? And what about retweets?

In 2009, an Illinois judge dismissed a case after finding tweets were a form of “rambling hyperbole” but courts have yet to define for sure what, if anything, you can’t say on Twitter.

The Phone Dog Case: Twitter and Company Property

Last year, a blog called PhoneDog sued one of its former journalists, Noah Kravitz, who took his 17,000+ Twitter followers with him when he walked out the door. Phone Dog says the journalist obtained the followers only because he worked there. It added the followers were a company trade secret worth $2.50 a piece.

The court case is caught up in procedural snarls but has in the meantime gone viral as a social media parable. For now, the law is unclear about who should get the Twitter spoils in a digital divorce between a company and its employees.

The PhoneDog case will surely lead other companies to amend their employment contracts. As for the parties, Kravitz is enjoying his day in the sun and PhoneDog, which tell its side of the story here, appears to be doing just fine without him.

The Religious Leader Case: Twitter and the First Amendment

In a story that should have received more attention than it did, the New York Times (NYSE: NYT) in December reported on a judge’s decision to dismiss cyberstalking charges against a Maryland man.

The man used Twitter to spray thousands of vulgar and threatening messages concerning a female Buddhist leader. They includes tweets like ““Do the world a favor and go kill yourself. P.S. Have a nice day”

The court found, however, that prosecutors applied the cyber-stalking law used to try the man in an unconstitutional fashion: “While Mr. Cassidy’s speech may have inflicted substantial emotional distress, [it is] protected speech: anonymous uncomfortable Internet speech addressing religious matters.” He compared Twitter to physical billboards used by pilgrims in colonial times and to an internet news board.

It’s unclear if these metaphors will gain traction. As in other legal situations involving Twitter, higher courts have yet to provide an answer of what type of speech is allowed. At the time, the woman’s lawyer said she was “appalled and frightened by the judge’s ruling.”

The WikiLeaks Case: Twitter and National Security

A year ago, Twitter made waves when it challenged federal demands that it hand over data about three account holders connected to Wikileaks founder Julian Assange.

Online media companies typically comply with these orders as a matter of course. Twitter, though, bucked this trend by suing to lift a gag order that prevented it from telling the account holders the government was after their data. The company succeeded which in turn allowed the Wikileaks subjects to challenge the government’s demands. The defendants ultimately lost but the case is still significant for media companies because, in the memorable words of Wired, “Twitter beta-tested a spine.”

Twitter became mixed up with another security and censorship controversy this week when it announced it has a tool to censor tweets at a national level.

The Tony La Russa Case: Twitter and Trademark

In 2009, the St. Louis Cardinals manager sued Twitter after a fake Tony La Russa account spat out tasteless tweets about dead pitchers and his DUI arrest.

The baseball manager, likely realizing he would strike out, quietly ended the case soon after. But the La Russa case became the first in a series of cases in which brand owners have tried to use trademark law to shut up Twitter users.

“Life settlement” agency Coventry First, for instance, sued to grab a Twitter account that was sending satiric messages about its ghoulish life insurance policies. Other big companies like BP appear to have decided its better to quietly tolerate fake accounts like @BPGlobalPr which continues to emit hilarious comments on BP’s efforts in the gulf (sample tweet: “Please, write your representatives and tell them you’ve forgotten about the Gulf of Mexico.”)

These type of trademark claims have since led Twitter to develop new ways to protect parody, in particular with its check mark system to authenticate famous people (but the system is not perfect as the wife of news baron Rupert Murdoch can attest).

What is the Significance of these Lawsuits?

If there is a common thread here (beyond Twitter’s ability to attract interesting lawsuits), it’s that established laws are not holding up under the weight of billions of 140-character messages.

Likewise, Twitter itself deserves some credit. In addition to protecting parody and challenging government security demands, the company this week decided to copy Google’s practice of displaying messages that are forced down by copyright law.

The bottom line is that Twitter is not just creating new forms of speech but helping shape rules for how that speech can be used.