I'm asked this question frequently by co-workers, relatives and loved ones and I usually fumble through a response that isn't exactly definitive.
But I work my personal twitter - http://twitter.com/alleverybody almost daily.
What do I post? It depends. But it ranges from what I'm hearing on the radio at the moment (I tweet a lot in my car but - no worries - not while I'm driving!) to what films I've seen to work projects I'm trying to figure out to silly philosophical comments to photos I snap/post with my phone. Simply, its the human side of me.
I also handle a couple client twitter accounts: http://twitter.com/IoPP_Pros and http://twitter.com/PSTC_Tape. The former is active while the latter needs more action (but this group is still - like many organizations - debating the "value" of social media). Twitter is a good way to dip your toe into the waters of social media.
I recently attended the Business Marketing Association's (BMA) 2010 Annual Conference (in June) and tweeting was quite - in a word - heavy. It was all over the place. People were tweeting during general sessions and breakouts. The conference had screens showing audience tweets as speakers delivered their presentations. Some of the tweets were action oriented but many merely restated what a speaker just said, which was good if somebody was following offsite. Bottom line - Twitter engaged immediately. And that's what you want - engagement with your audience with a sense of "you need to know this now."
Not sure about Twitter? Chris Brogan (who spoke at the BMA conference), president of New Marketing Labs, is a true believer in the power of Twitter. If you need that little push off the cliff into the deep waters of Twitter, Chris gives you 50 ideas on how to use it here.