There's a technical problem with this. Dropbox syncs between multiple clients. That means multiple clients can change a file. IF the files are encrypted and the service doesn't have the key, there's little the service can do about a file that's (effectively) simultaneously changed on two machines, that then try to sync.
You could zip up your files, encrypt them as strongly as you want and then upload them to some server somewhere at any time (Say you get a hosting account) and then nobody else but you has access to your password, and presumably that would be a solution you seek-- but maybe not the same level of convenience.
I think the convenience (depending on what you want) is intrinsic to the lack of security.
I don't understand what technical problem you are referring to. Dropbox doesn't marge files. If two computers change it at the same time you get two different files and you have to manually fix it. The only thing dropbox needs unencrypted is the set of files you make public.
I'm not sure exactly what technical problem you're alluding to, but it is possible, albeit very difficult to perform computations on encrypted files without the key.
You could zip up your files, encrypt them as strongly as you want and then upload them to some server somewhere at any time (Say you get a hosting account) and then nobody else but you has access to your password, and presumably that would be a solution you seek-- but maybe not the same level of convenience.
I think the convenience (depending on what you want) is intrinsic to the lack of security.