Ten1 Design has a new piece of software that can turn the Macbook Trackpad into your own miniature Wacom Tablet for only $24.95.  Install “inklet” on one of the newest unibody Macbooks that support multitouch and OSX 10.6 (Snow Leopard), and you’re good to go.  A free version is available to try it out.

Inklet Trackpad Tablet.