Great concept, I found it fun! Excellent use of one button gameplay.
However, for a game this precise you should probably not use time delta. I was getting a lot of fluctuations of fps and therefore ball bounce height on my iPhone 4s and it was frustrating to miss a bounce because of it, which was pretty often (then again, perhaps the frame rate fluctuation without time delta could mess with the timing and might be even more affecting of the gameplay. I dunno, I guess it would require play testing).
Also, there was a bug at the end of the fourth level right before the last spike where the ball disappeared - it wasn't destroyed by a spike - and I couldn't complete the level.