Jon's keyboard makes a kinda bright piano sound, you can hear the sound it makes on the "revolutionaries wait for my head on a silver plate" part of viva la vida.It's true he doesn't play the strings, but sound is coming from the keyboard. [ame=http://www.youtube.com/watch?v=VX1eJHE1K_s]Viva La Vida Coldplay live at the BBC - YouTube[/ame] when Chris comes on he tries the keyboard-that is the sound. :) FYI They don't intend to make you think they are playing everything live.
UPDATE: THE SOUND IS A TACK PIANO THE SAME AS IN LOVERS IN JAPAN