One of the most serious problems with Spotlight is that you cannot search for phrases in document content. One solution is to troll through the results of a Spotlight search using other utilities to do the phrase matching.
I wrote a python script to demonstrate this. mdgrep.py "frank zappa" will first search the Spotlight index for all documents containing frank and zappa. It will then extract the text using mdimport -nfd2 to take advantage of Spotlight plug-ins. The script has command-line options for limiting the search to specific directory hierarchies, and searching using regular expressions. The output of the script is a list of all files with the given phrase.
You can always find the latest version of the script on my site (the first link above is hosted on macosxhints.com). This was tested using python 2.3 and 2.4, and obviously requires 10.4.
[robg adds: I tested this script by grabbing a phrase ("wrong one more than once") out of an old Macworld column, then searching for it using Spotlight and mdgrep.py. Spotlight found the right document, but it also found about 15 other columns that contained those very-common words. mdgrep.py found just the one document, but it took a while -- over 10 minutes on my Mac Pro. Execution time would have been much lower if I'd restricted the search to only my Macworld columns folder, but I wanted to see how it did with my whole system.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070405115719975