As of MacOS X 10.4.1, it appears that all of Spotlight's resources related to the menubar item (such as its menubar icon) are stored in:
/System/Library/CoreServices/Search.bundle/Contents/Resources/
If you don't want to manually dig to the location then use the Finder's "Go to Folder" feature under the Go menu, paste the above path in, and you're there! Have fun, and remember to backup before going nuts!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050528081106743