Earlier quoted context omitted.
This is assuming the Government doesn't force Dropbox to install a keylogger on their client to get users TrueCrypt passphrases and possibly keyfiles.
If Dropbox bundled a keylogger with the Dropbox client, I'm pretty sure it would eventually attract some attention...
The underlying problem is that you're trusting closed client software from a third-party. Once that is running on your system, arguably it's game over as far as what Dropbox or its like can and can't do to you.
(If you trust them, ok. But when documents state that "they're next", many of us start to wonder. And again, this isn't just access to a server. Through their client software, this is access to your machine, that you've already granted.
Admittedly, I'm describing the typical Dropbox user scenario and not where the client is manually applying -- or not -- each update, and somehow supposedly gaining insight into the capabilities of each update/version before doing so. And I'm also ignoring various forms of restricting the access of the Dropbox client, although if they really want you, they and parties using them would have extensive resources to apply to breaking out of such constraints.)