A caveat here...

Say you carve out prior inventions, existing knowledge, exception to non-competing projects, etc. You think you're safe... BUT you ask your employer to reimburse your high-speed internet connection. All of a sudden, they can argue that you are using company resources (though off-hours) and therefore even if the project does not overlap, it's the company's IP. Same thing with using your (company-paid-for) iPhone to do testing, business negotiation, etc. I may sound paranoid and I don't know where to draw the line. I think everyone ought to be careful about this sort of things.