I hadn't thought much about the security of it, but when listening to Steve Gibson and Leo Laporte on the Security Now podcast (http://www.grc.com/securitynow.htm, or search for it on iTunes) examining Dropbox, I got an eye-opener that you can't assume an awesome service to by definition have awesome security.
First of all, Dropbox have claimed that not even their employees are able to see your data. Great! But in a recent change in the terms of agreement it says that the authorities due to US regulations can ask Dropbox to decrypt your data in certain crime investigations. Allright, I'm a good guy so that's not a problem for me. But that means that Dropbox must keep my encryption key in their vaults instead of me doing a client side encryption/decryption of my data. Interesting, that means that a bad apple Dropbox employee also have the possibility to look at my data without my knowledge. Not to mention what would happen if Dropbox would lose the table of private keys in some master planned hacking or insider heist.
So, the lesson should be. If you have some valuable or sensitive data, you should probably encrypt it before even dropping it into Dropbox.
Well, that applies to companies or people with more valuables than family photos like me.
Issue two might be more concerning, Derek Newton, http://dereknewton.com/2011/04/dropbox-authentication-static-host-ids/, has looked into how your Dropbox client authenticates against the cloud service. It seems like all you need is a config file which is set up at install time. That file contains your hostid which is your authentication token against Dropbox. The bad thing is that if someone by social engineering, a trojan or other malware gets a copy of this file, they can access your Dropbox account from any machine. Changing your password is not enough since this is an access token. You must remove your own machine as a valid host from Dropbox to stop the bad guy from using your account. Most probably you won't even know someone is eavesdropping on you.
These guys also seems to trust the cloud a bit too naively