And you owe taxes on it, regardless. I heard some sob stories from the brokers at Charles Schwab after Enron went under - energy traders who had borrowed heavily (on margin) against positions that were now worthless. Oy.
Some of the worst horror stories are from people that exercised their options and then waited a year to sell the stock to qualify for long term capital gains. Then the stock subsequently crashed so they owed a massive AMT tax for the difference between strike price and price on exercise and got almost nothing from the actual stock sale.