Ok, this should clear it up for you:
Spotlight is the new metadata technology in Tiger (I'm sure you're aware of that). Spotlight is able to index everything, or just some things, as you wish. Spotlight technology can be utilized by developers in their apps too with plugins. The Spotlight icon in the menu bar searches through all of your file types: documents, folders, mail, chats, etc.
However, each app can utilize Spotlight too. So, the Finder search now utilizes Spotlight so it can find files and folders instantly. In Mail, you can find mail messages instantly. Same for any other app that takes advantage of Spotlight. I'm not an Adium user so I don't know what search is like in Adium, but the developer(s) behind Adium can take advantage of Spotlight in Adium and have Adium find text in chats instantly now.
Hope this clears things up.
-Chase