It depends on the state. In most states If you are recording someone without their knowledge you are violating the wiretapping law and may be subject to fines and/or even jail time, depending on the state law, or you can expose yourself to a civil lawsuit (see California wiretapping laws, as an example of state law).
Maybe somebody should have given Linda Tripp a "heads-up" when she decided to tape her telephone conversations with Monica Lewinsky. Sure, she did a lot of damage, but in the end she lost practically everything she owned.