Many players have been playing with negative cash lately as an easy credit form to jump start their company expansions. In reality when your account balance is overdrawn you'll end up paying interests.

Of course this game has an overdraft protection, not allowing players to purchase goods over their cash limits, no building of new units when you don't have cash and so on. But it still happens, when you pay taxes, or employees that you end up with negative cash. That's why there now is an indicator on the home page warning you if you have enough cash to finance your taxes or if you'll have an overdraft after taxes.

In the future you'll have to pay interests on that negative balance. Forcing players to keep their cash flow in mind. Loaning will be be a way to fix a cash shortage when you are about to overdraw, since the long term loans will be much cheaper. So there is still a way out, but more skilled players will save up cash for expected expenses.

More information about overdraft:
http://businessgame.be/help/faq/83