How dumb is that? Now, Letterman and his ilk are comedians and I find them to generally be equal opportunity in whom they lampoon, but that crack needs to be addressed. Why? Because it contains a popular ignorancy, that's why.
Who classifies documents? The likes of the CIA, the FBI, the military etc. Which branch of government do those organizations serve? The executive branch and what the executive branch makes secret the executive branch can make unsecret. President Bush is the top executive and the authority to make documents classified or unclassified is ultimately up to him! Now, most of the time it is quite obvious President Bush and his immediate subordinates do not bother themselves with the classification and unclassification of documents, that is a job they have delegated. None the less, it is ultimately up to the President and those he authorizes to classify and unclassify documents.
To sum up, to say President Bush leaks classified documents is an oxymoron.