The accounts of 55,000 Twitter users have been hacked and the usernames and passwords copied into Pastebin. The hack is the largest in the service’s history, and required five pages on Pastebin to leak all the data. Twitter have said only that they are looking actively at the situation.
The ease with which the service could be penetrated could make users, and potential users, more cautious of using it, although I believe the idea was to highlight security flaws as opposed to anything more malicious. As AirDemon point out, this could have easily been avoided, by guiding people to use more secure passwords when they signup as is common practice on many websites, and encouraging regularly password changing. Twitter has always emphasised how easy it is to sign up and start posting, but this has left its users’ data vulnerable.