Here's my opinion:
START and SELECT are generic and mapped however any program wants to use them. In an emulator they are exactly what they say. In a web browser, SELECT could switch tabs if multiple pages are open. These buttons can be anything to any program.
The MENU button (now considering a Pandora logo, how cool is that?
) would be tied to the OS. Obviously, hitting it should not automatically kick you out of anything because mistakes happen but it could bring up a task manager that you can use to switch between tasks, kill tasks, go into standby (power button can do that too), enter a kernel menu with all kinds of stuff (like what to do when lid gets closed of power button cycled) or just cancel and go back to the current task running. To me that seems really handy, what do others think?