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.
There may be broken links to media on this page. I'm still in the process of migrating them over.