I have the awnsers.
Chris Martins piano is a real piano. BUT they have installed midi-triggers under the hammers like a digital piano or keyboard. I know this because there is alot of other bands that have done the same. Keane use a Yamaha CP60 wich is a real piano with both hammers and string. but they have installed midi triggers so he plays organ in a song called Hamburg Song. They have also some Rhodes keyboards and they use them for acoustic sessions but it sounds like a grand piano.
Or it could be like this...
The piano he uses live is a real piano with pickups. I have for example a Yamaha CP80 wich is also a real grand piano with both hammers and strings, but it has pickups under every string, just like a guitar. so i can plug it into anything just like a guitar. and when i do that,I CAN change the sound to anything i want. there is alot of technology that can change sounds or even emulate other instruments.