Win7, GTX 560 Ti, GTX 970
Broken: 2.72 official 64bit, current buildbot: Hash: 9590e77
Short description of error
Currently the Cycles uses clipping sphere instead of clipping plane and Freestyle uses clipping plane as expected but when using Freestyle with Cycles this causes weird behaviour.
I strongly suggest to change the behaviour of Cycles and leave the Freestyle as is, because the clipping plane is far much easier to work with and is also supported in the OpenGL viewport.
You can see the problem in the attached blend. If you render the cube, cycles can't see the monkey inside, but freestyle can. It would be great if cycles coud see inside the box as well...
Exact steps for others to reproduce the error
Open the file and hit Render. You can see that the monkey is partly hidden for the cycles but fully visible for the Freestyle.