Leopard Mail.app has unique message identifiers

This will enable many very important productivity applications.

Daring Fireball: 'message:' URLs in Leopard Mail:

... The new version of Mail in Leopard introduces a ‘message:’ URL handler that allows you to refer to individual messages in Mail from other applications. You can use a utility such as RCDefaultApp to see that Mail registers as the default handler for the “message:” scheme. That Mail now supports these URLs does not seem to be documented by Apple anywhere, but it’s fairly simple, and very useful. It’s one of my favorite new features in Leopard Mail...

It qualifies as a 10.5 "hidden feature".

Now if only Mail.app (and Gmail) would implement Outlook's most important feature -- the ability to edit the subject line of a received message.