As we know, the iPad uses the A4 processor. However, A4 is a pretty arbitrary name, right? It may just be the middle of a family of Ax processors, as follows:
A1: iPod Nano
A2: iPod HD/iPod Touch
A3: iPhone, maybe iTouch
A4: iPad
A5: MacBook Air??
I believe it has to do with the number of cores used as well as it being twice as fast and efficient that the current A8 processor in the 3GS. But I don't think it has anything to do with a sequence of processors for different devices. I thought I read somewhere they will be using and A4 processor for the new iPhone as well.