Magicprefs will do this for you. It's free and works well most of the time. It actually has a bunch of settings you can change and tweak. One of those settings is a middle click and it allows you to change the hit zone so you can make the middle click really large or small depending on what you want.
Someone mentioned above jitouch which handles the mm trackpad and the magic pad. I haven't tried it but it looks interesting. Not free though.
Edit: I just tried out jitouch and it works with magicprefs as much as I use it for. I only use magicprefs for the middle click function and the tracking speed. I really like the tap to switch tabs. It's a well made application and only 6 bucks. I think I am going to permanently use it now.
Someone mentioned above jitouch which handles the mm trackpad and the magic pad. I haven't tried it but it looks interesting. Not free though.
Edit: I just tried out jitouch and it works with magicprefs as much as I use it for. I only use magicprefs for the middle click function and the tracking speed. I really like the tap to switch tabs. It's a well made application and only 6 bucks. I think I am going to permanently use it now.