I have worked for small businesses since the start of my career, and before that in college. And I have noticed a glaring loophole in Romney's argument that cutting income tax rates on businesses will spur investment and hiring:
Businesses are not taxed on all of their revenue. The income tax rate on business is only on their profits. In other words, a business is only taxed on an investment if it is already making money off the investment.
Under no circumstances will an increase in the rate of that tax make what would have been a profitable investment turn into an unprofitable one, because the tax doesn't come into play unless the investment is profitable in the first place.