How to enable the "middle" button of Apple's new Magic Mouse?

Here are two utilities that will allow you to customize gestures on the Magic Mouse, including adding middle click:

MagicPrefs

Better Touch Tool

If you are using the middle click just for opening links in a new tab in Safari, you can do so by pressing cmd while clicking.

Try this software: middleclick

Works really well for me on my 2 year old G5 running 10.5.