Recently I noticed an article about Dropbox – http://www.wired.com/threatlevel/2011/06/dropbox/ (Update: the URL seems not to work any longer. Also see here: http://www.wired.com/2011/05/dropbox-ftc/).
The problem: With Dropbox data will not be encrypted on the computer of the user, but on the servers of Dropbox – so besides Dropbox also potential invaders can gain access to all data stored there. One result was now, that on last sunday for four hours every data of all Dropbox users was freely accessible!
There are better alternatives, like http://www.wuala.com or https://spideroak.com, where the encryption is done completely on the users side and an invader can not do anything with the data on the servers.