I could find articles and patents about this, but they all seem to use a special shaped piezo element.
Do you want to use a normal piezo element ?
The Arduino could use a digital output with a certain frequency, but that would not be a sine. How would you want to read the resonance frequency ?
Take a look at the ping))) library : http://arduino.cc/en/Tutorial/Ping
The piezo needs extra hardware to do that.
In the same way you need extra hardware for your piezo sensor. Controlling that extra hardware is what the Arduino is good at.