Many pianists - and not only pianists - sing along when they play; sometimes it's more like groaning. It may help the pianist to phrase melodies, to get more "into" his play. Famous examples are Glenn Gould, Keith Jarrett, Bud Powell and Errol Garner. Undoubtedly there are more. I know I do it. Studio engineers make the greatest efforts to isolate the sound of the acoustic piano from the groaning. In the first link you here Errol Garner in Earl's Tune. In the second link you hear Herbie Hancock in the studio, while recording Stitched Up. In the third link you hear Glenn Gould.

http://www.youtube.com/watch?v=GS5S0d_JLKQ#t=0m23s

http://www.youtube.com/watch?v=Lgwze5CkZAI#t=0m33s

http://www.youtube.com/watch?v=thk_Xap-Isw