Can you go to jail because of your tweets?

Few days ago, South and North Korea had a military confrontation. Amidst the flood of news articles on that subject, one article caught my eyes. It was saying that the South Korean police is arresting anyone who are spreading false rumors.

A few people were arrested because they either send cell phone text messages to random people or wrote on their social network services (such as Facebook wall or Twitter) false information such as Kim Jong-il is dead or North Korean army officially invaded the South.

There have been some people saying because of the wide reach of technology and especially wide reach of the social networking services it has become so easy for rumors to spread out. There was a study saying that most people spread the rumor over Facebook or Twitter without even checking whether the rumor is true or not.

So, here's my question. If I get arrested based on what I post on my Facebook wall, is that a violation of my freedom of speech? Also, am I responsible to check everything I post is true? Are my posts weigh as important as official news articles from professional journalists (like those work for CBC or CNN)?

PS Here's a Washington Post's article on a similar topic: http://www.washingtonpost.com/wp-dyn/content/article/2010/11/23/AR2010112302121.html?wpisrc=nl_tech

PS Here's a Telegraph's article on a similar topic as well: http://www.telegraph.co.uk/news/worldnews/asia/china/8143878/Chinese-woman-sentenced-to-hard-labour-after-satirical-Twitter-comment.html