This post on Twitter’s blog explains the rationale behind extending twitter characters to 280 and the results of their test.
In a nuthsell:
“During the first few days of the test many people Tweeted the full 280 limit because it was new and novel, but soon after behavior normalized (more on this below). We saw when people needed to use more than 140 characters, they Tweeted more easily and more often. But importantly, people Tweeted below 140 most of the time and the brevity of Twitter remained.”
How do you think things change will influence your Tweets?