"Back in the day" ... electronic whiteboards were wonderful for
presentations because of how easily one could make ad hoc notations as well
as be able to answer questions by writing on the original presentation.

Doing presentations using a laptop and a projector is a BIG improvement
except in making ad hoc notes on the presented page, saving them, and so
on.  You can point to things with a cursor or laser pointer (which lots of
folks might not see see) but doing so cannot save the point for you to
update the presentation.  I'd like to be able to draw on my screen, have
that show on the projected screen and have the option to save that annotated
slide for myself.  Sounds like a tablet PC type of thing to me.  This is
what leads me to ask:

   - Is there a Linux (or Linux utility) that lets the user use a tablet
   PC in that manner?
   - Is there any real downside to such a system (e.g. inherently
   underpowered, etc)?

Note, that using a graphics tablet does not sound good for a couple of
reasons, not the least of which being the difficulty of relating the touch
point on the tablet to the content of the presentation.  Writing on one is
even worse.
-- 
The nice part about being a pessimist is that you are constantly being
either proven right or pleasantly surprised.
  - George F. Will