it's called Inkwell and is part of OS X .
You can write using the trackpad, the built-in one or the Magic Track pad, the stand alone one.
It requires 10.6 but that is not a problem in your case as the MacBook Air will have that when it arrives.
I have an iPad and use PenUltimate as a Handwriting Notetaking app.
My fiancee was going to get an iPad, especially for use with the iPad. When the MacBook Air came out she decided to get that instead.
She will use it a lot for typing but she's wondering if there's an OS X application that using the Magic Trackpad as a writing tool where she could take notes by handwriting with her finger on the magic trackpad. Is there anything like this?
Thank you for your help.