Tor is already vulnerable to anyone that can watch a large enough portion of global network traffic. Thanks to the NSA taps on US internet use & the (recently revealed) taps that GCHQ puts on EU internet traffic travelling through the UK it seems likely that the NSA can track Tor users in the US and most of Europe in real time if they choose to.
(It's fairly likely that the US has covertly tapped undersea cables elsewhere in the world as well.)
(It's fairly likely that the US has covertly tapped undersea cables elsewhere in the world as well.)