Dark Wallet coinjoin goes through a server, but the server cannot see any details of your transaction nor can they steal your coins. All details are encrypted for the counterparty, and all signing happens in the client.
The only weakness here is that a server gets taken down stopping the service in which case you switch to another service. Even if the NSA controlled the server, they wouldn't be able to steal your coins or observe your transaction at all.
Lastly the server is sharing messages with other servers (we are improving this too), so it isn't really centralised. It is federated kind of like how different email providers inter-operate with each other. The decentralised aspect will only improve over time as we develop standards and deploy technology.
Are you guys running a server?
I'm not a laywer, but:
If that is the case, that server could be shut down and Mr. Wilson and cohorts arrested for "running a service" that "facilitates" or supports illegal activity. Its clear that DarkWallet will be attractive to people who are doing things that are considered 'illegal'.
If I'm not mistaken, Liberty Reserve and eGold were shut down for the same reason. Liberty Reserve's founder is apparently facing a long prison sentence (the charges against him include his boastings about the service's illegal utility).
The developers of Bitcoin, and BitTorrent for that matter, don't face legal liability because they just write the software. They don't actually operate anything.
I like DarkWallet, but hopefully the 'decentralised aspect' will improve very soon. The us is obviously going to do some very intensive traffic analysis on whatever server(s) you're operating.
It's all encrypted. Try proving anything.