I did exactly this when I joined justin.tv - it's not very difficult to arrange. The hardest part was actually finding a CPA who had any availability to work with me, and who I felt I could trust (he came recommended by a friend).

It's obviously important to keep in mind that to exercise your options you'll need some cash. If your share is big and the company has been well funded already, you might need a lot of cash. And you need to be prepared to lose it all if the company fails.