Why do you assume the A4 is based on ARM? PA Semi, the semiconductor company Apple purchased back in 2008, specialised in low power POWER/PowerPC designs.

I grant you, running ARM based iPhone apps on the iPad would be sub optimal, but Apple has plenty of experience running non-native binaries on their hardware (see Apple Rosetta for running PPC software on x86 hardware) and for many applications the performance hit may not even be noticeable.

I'm certainly interested to find out more about the A4 though, it should be interesting.