Spotlight was likely showing those .plist files because they were located outside of the library/preferences folder. If they are in a location considered to be "System files" then you'll need to use the aforementioned technique.

Great info Halitosis, especially about option-clicking the + button to get extra filtering options.

Update: I just realized that Spotlight will find all .plist files without adding the "System files" criteria, but only if you ask it to look in the ~/Library/Preferences folder. In other words, navigate to the folder that you want to search first, then type keywords into the search field in the Finder window.

Now that I'm starting to understand the nuances of it, I can see that it is more mature than Tiger's search functions. I like it.