UPDATED: Search-Replace Tag Text v3.0
Search-Replace Tag Text performs a search-and-replace with the text in your choice of tag (Song Name, Show, Artist, Album Artist, Album, Composer, Comments, Genre, or Grouping) in the selected tracks or all the tracks in the selected Playlist.
This latest version is written as a Cocoa-AppleScript applet for OS 10.6 and 10.7 only and features case sensitive and whole word matching, as well as a "dry run" mode that will display a list of what will be changed before you actually commit to the search and replace.