Rob’s suggestion does work (and this is how I implement onUpdate for my apps) but it’s quite cumbersome having to manage adding and removing event listeners in code for something as simple as this, especially when you have multiple tweens requiring onUpdate.
Most modern tween engines (JQuery, Greensock, GTween) support onUpdate/step and long ago even Adobe’s in built AS2 tween API supported this so why can’t the Corona SDK?
Here’s the link to Caleb’s feature request if anyone else wants to up vote this: