MacOSXHints recently figured out that Leopard Mail has some helpful searching tools. Put simply, some of the improved spotlight functionality has found its way into the mail search code.
The three tags that are known at this time are from, subject, and email.
This command would get you copies of all emails from me.
This would return all the emails with MacSparky in the subject line.
This returns all the email from a specific email address (as opposed to the identifying name tag).
The full article is at OSX hints right here. It is strange they don't include the boolean operators (which you can use to search email through Spotlight). It is also strange, but not unusual, for Apple to include this kind of code and tell nobody about it. Doesn't it make you wonder that some software engineer might have just put it in for his/her own interest and not bothered to tell anyone about it?