I took some time out to analyse over 100,000 tweets about the Japanese tsunami yesterday. Although I haven't properly looked into the data yet, the levels of sentiment are incredibly interesting.
Below is a graph of the twitter sentiment across the majority of the day in the UK. Click on the image to view it at a larger scale.
For the record: I only started tracking the twitter sentiment after the tsunami had hit Japan. The following graph is the reaction to the event on Twitter.