The Apple iPad specification says things like "Processor: 1GHz"
According to Wikipedia, iPod touch RAM is now 256 Mbyte, so it's reasonable assume an iPad is no worse.
An Arduino's ATmega runs at 16MHz, and program memory 32 Kbytes with 2 K RAM. It isn't really comparable.
What software would you run? Linux maybe?
If you are interested in pursuing this, I think you might start by looking at something like fast ARM-based machines, AMD, Intel, VIA mobile processors.
A low-ish cost starting point might be a Beagle board
or maybe, just about, an Atmel ARM
There are lots of ARM-based machines in the mid 0.5GHz performance area that you could investigate.
If you are very interested, you might look at Atmel's (the company who make the microcontroller for Arduino) QTouch and MaxTouch technology.
You might find the company who makes the touch screen for Apple, and see if you can find someone who will sell it ready made.
Of course, you could wait a few months to see what companies like Asus bring out.
IMHO, making an iPad-like machine is a non-trivial undertaking.
HTH
GB-)