Many would need to be recompiled anyway because of the different screen size. Of course it is entirely possible to write an iPhone app to work correctly whatever the screen size is, but many developers won't.
True. (Unless they keep the same resolution but bump the screensize.. shudder!!
Not at all. Nobody writes code for an ARM processor. People write code for C, or C++, or Objective-C. Compile it and it works. The iPhone simulator is actually running plain x86 code and using MacOS X libraries. I bet most iPhone developers have not even checked personally whether there is really an ARM processor in the iPhone or not. Programmers don't even care whether their code runs on a CPU or a GPU, why should they care which kind of CPU it is? Nobody is rewriting any code.
True. The only possible exception is performance optimisation - if the tablet runs the iPhone OS on an entirely different h/w architecture, it's possible that performance in different areas varies greatly.. Perhaps some games might need graphics/poly count to be scaled back if the tablet is slower, or it's even possible that some apps might run into timing issues if the tablet is much faster (none that are well written though!