And sure, this could be an email (all of the same principles would apply), but a letter makes things official in every sense. It also gives you a prop to take along to the face_to_face meeting with your boss, when you let him or her know that you're leaving.
State Your Intent. The whole theme of your resignation letter is the statement that, well, you resign. You don't need to play coy__the time has passed for you to fish around for a counteroffer if it's gotten this far. At this point, you've made your decision to leave, and this letter needs to reflect that.