There has been a lot of speculation, but it is all that, speculation.
With the Russian control of manned access to the station (Only Soyuz carries humans today in 2014) and the majority of control of cargo (Progress, and Kurs docking computer systems used on ATV) they have a pretty strong position.
Also, only Progress and ATV can deliver fuel to the station, Dragon, Cygnus, and HTV which berth on the US segment cannot deliver fuel to the Zvezvda module for reboost uses.
The US would be foolish to push on this issue. Conversely the Russians would be foolish to try and use this for leverage, since they cannot afford to run the station on their own either. Sort of mutually assured destruction if either tries to use the ISS as leverage.
Come 2015/2016 if SpaceX can deliver on Dragon manned (DragonRider), Boeing with CST100, SNC with Dream Chaser, then the story can change.
For now, all sides seem to be keeping their heads down on this issue, so no good comes of it, in any direction. But that being said, politicians have done stupider things in the past.
Having said that, assuming things went horribly south, an interesting question would be whether the US would be willing to launch humans on a Dragon, without the escape system in place.
Right now, Elon Musk has strongly suggested that should a Soyuz develop a problem that it could not reenter, a Dragon could be launched in Cargo config with seats, to safely return a crew. Whether the US would be willing to launch on it is a different story.
Of course, it would be a great idea to get Dragon/CST-100/DreamChaser ready as soon as possible.