I've noticed recently that Amazon has started listing some significantly improbably phrases for many of their books.
About a year ago, my sister Jenni and I wrote a Python script to do something similar (although only at the word level, not phrase).
Inspired by Amazon, I've now put our script up at http://jtauber.com/2005/03/z_value.py
I'll need to think a little more how to extend it to phrases. In the meantime, have fun with the script and let me know if you have any suggestions.