Given the NSA's extensive tapping of key fiber lines, we should assume they can actually observe the necessary traffic.
From the paper: "A global passive adversary is the most commonly assumed threat when analyzing theoretical anonymity designs. But like all practical low-latency systems, Tor does not protect against such a strong adversary." --- Tor: The Second Generation Onion Router [0]
[0]https://svn.torproject.org/svn/projects/design-paper/tor-des...