From everyone's favorite lesbian financial guru, Suze Orman:

"Also, never ever borrow against your 401k plan because you will pay double taxation on the money you borrow. Because you don't pay taxes on the money you put into a 401k, when you pay back the loan (which you must do within five years, or 15 years if used to buy a home), you pay it back with money you have paid taxes on. Then, when you retire and take the money out again, you end up paying taxes on it a second time. And that isn't even considering the penalties you have to pay if you change jobs/quit/lose your job, in which case the money is due immediately and subject to taxes and a 10% penalty."