at the expense of conveniences like web access, document previewing, simple sharing, etc. - sure :-). if your answer to the web access concern is: derive the key from the password, who's to say we wouldn't store the key and later use it to decrypt your data?

web access non-withstanding, you'd be making a leap of faith to believe that the client is 100% trustworthy and that encryption is actually happening. at some point you have to make a decision as to whether or not you trust the entity (dropbox, google, or anybody else). if you don't, you should use something like truecrypt between you and the service.

all arguments made against dropbox apply to your gmail attachments, gmail mail, google docs, etc.