I've noticed that Mail in Tiger has a new feature when you're selecting text, in that you can scroll while holding down the mouse button to extend your selection. A very useful addition and much lacking considering it's been in Windows since at least 2000. This behaviour is also to be seen in the Finder, also very handy.
However, this doesn't seem to be standard behaviour. It doesn't work in TextEdit or Pages unfortunately, so it looks like it's only being implemented on a per-app basis. Let's hope they add it to all apps without any changes having to be made to the app in question.