I have found this find command useful to find anything in a selected directory and open the matching items with the application set to open those file types. It will look at file names and the contents of the file in order to find matches:
sudo find /path/to/dir_to_search -type f -exec grep -i -q stuff_to_find "{}" \; -exec open "{}" \;
Replace /path/to/dir_to_search with the full path to the directory in which you'd like to search, and replace stuff_to_find with the string you wish to match.
[robg adds: This works, but be careful with what you're searching for; if it appears in a lot of files of different types, your machine could be busy for quite a while, launching apps and opening files! You don't necessarily need sudo, if you're searching within a directory you have full rights to use. Also note that if what you're searching for contains a space, you'll have to do some work with quoting and backslashes to capture it properly (examples in comments are welcomed!). If you want to simply run the command at or below the current directory, replace the /path/to/dir_to_search bit with a single period (.); this will search at and below the current directory.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050413170530614