One day I was traveling through CLT airport, and I had a fairly lengthy layover. In the central courtyard area, they have that nice grand piano that I have heard being played many times. I play a little jazz piano, so I didn't think much of it when I sat down to play a few tunes, but after about 8 minutes, someone came by and was very cross with me for having done so! I was told it wasn't a public piano. This got me thinking about where there ARE public pianos in airports that anyone can play. Here are the ones that I have played/know of (all US domestic... I haven't traveled as much as some):

CAE
STL
PDX (granted, this was in 2012... not sure the piano was temporary or permanent)
RDU (also not sure if it was permanent)