One of the strongly emphasised features of Dropbox is deduplication (i.e. so you won't need to re-upload massive files if someone else already has), and if you are particularly security conscious, presumably you would have made the effort to read about how Dropbox stores their files and considered whether this method was adequately secure for you prior to use.
I'm assuming the single key is related to deduplication, and although I don't understand why you couldn't simply store a key per user, I'm not even sure whether that would be more or less secure given that insiders with access to the encrypted files can probably also get access to the database of encryption keys.
As far as some select employees having access to your files, surely this is to be expected - the alternative is they get sued/charged by the authorities and are forced to implement an even more compromising solution than a couple of Dropbox employees having access (e.g. a backdoor for the RIAA).