I have recently created a Piano app which is available in Android Market as LittlePiano (free app).
It has 13 keys with multitouch interface.
The multitouch seem not to work great for two fingers nearby on HTC Desire - it is much better when they are not touching nearby keys.
Also - there is a "follow me" guide with few sample melodies.
I would highly appreciate a short test report on how it works on other devices.
Also if you want you can share your song (as long as it fits in 1 octave) using the following notation:
CDEFGAHV (this are the white keys)
cd fga v (this are the black keys)
please provide a title and if it seems a valid melody I will add it in some future releases.
Thanks in advance for all test reports!