The hackers are also threatening a database leak: https://twitter.com/1775Sec/status/421822727331131392
EDIT: Dropbox's statement is that it's maintainance issues: https://tech.dropbox.com/2014/01/dropbox-status-update/
EDIT2: There is a high probability that the Twitter account is faking the hack, due to the "proof" of the hack being taken from an old dump.
EDIT3: The account admits they did not hack Dropbox, just DDOSed it. https://twitter.com/1775Sec/status/421848589480910848