Is there a presentation mode on mac?
The best 3rd-party based solution for you is to use Caffeine.


  About Caffeine:   A tiny program that
  puts an icon in the right side of your
  menu bar. Click it to prevent your Mac
  from automatically going to sleep,
  dimming the screen or starting screen
  savers. Click it again to go back.
  Hold down the Command key while
  clicking to show the menu.