If the user is using Tableau Desktop, then he can click choose presentation mode (by clicking on the Slide looking icon next 'Abc' on the menu shelf or going to Window (main menu)-->Presentation Mode). Otherwise, there if the user is viewing the Dashboard via Reader or Tableau Server, this is not currently possible. This might be a good one for the Ideas section though.
Hope this helps!
Thanks Tracy - this is as I expected but am disappointed to hear confirmed.
Thanks for your time.