A way to use iPad as a sketching device during presentations (without cables)?
I assume that you have your presentation or document you want to show on your Mac. If so, the setup I use is as follows:


Install AirDisplay on your iPad and its client on your Mac
Install CocoPad on your mac.
Using AirDisplay "link" your iPad to your mac and mirror the displays.
Connect your projector and mirror the display
Create a new document in CocoPad and in the right corner choose a "transparent" canvas.
Bring the  transparent canvas on top of the document you want to scribble on and then scribble with your finger (or a stylus) on the iPad


Since AirDisplay relies on a wireless network to become an external display, you may want to make your setup more reliable by creating your own wireless network. To do so go to System Preferences-> Sharing -> Internet Sharing and enable it. Connect your iPad to the "created" wireless network and the connection will now be rock solid.

Another option that you could try out is inklet with cocoPad but it is a bit more expensive but with that option you would not need an iPad.

Hope this helps