Apple's A4 chip is nothing more than an ARM Cortex 8 based chip, a single-core and it looks a lot like the ultra popular Snapdragon.

The chip is definitely not dual-core as Apple would have certainly use this fact in its press event as an advantage. This is what a few sources confirmed and this is how backward compatibility comes into the game.

If you have Nexus One or HTC HD2, you will have as much horsepower as with an iPad. This oversized iPod touch is responsive, reactive, and fast as it can run only one application at the time. HTC for instance can run tens of applications simultaneously before it runs out of breath.

HD capability is something we need to investigate but Apple implies that the device should be able to play 720p videos. Intel solves its HD incapability with a Broadcom decoder chip, but we still don’t know what is the case with Apple's device.