WebSockets protocol is a tricky beast to evaluate right now, as it's changing frequently. After the flaws in draft-hixie-thewebsocketprotocol-76 of the WebSockets protocol discovered by Adam Barth et al. a few months ago, Firefox disabled WebSockets implementation in about:config preferences. Since then, a new protocol version is in the works, currently up to draft-ietf-hybi-thewebsocketprotocol-07, that tries to fix the flaws discovered. The browsers quickly adapt to the new version - see e.g. Mozilla bug #640003. But - on the other end, WebSocket servers still try to maintain backward compatibility, e.g. Socket.IO server still supports version pre-76.
So you might still succeed in trying to connect to WS server outside the browser, forcing older version of the protocol (see my tool already mentioned in the comments above).
But still - you are unable to send completely arbitrary traffic through WS connection. The frames begin with NUL byte, end with \xFF, with UTF-8 string inbetween (+ a handshake), so cross-protocol-attacks are not so easy to perform. Still, I'd rather use WSS:// encrypted version than the plaintext one.