The fees are already implemented, in the case of very large transactions.

It is done so that when you create a transaction, you put more bitcoins in than what goes out. The difference goes to whoever manages to create an accepted block with your transaction. If I wanted, I could modify my client so that your transaction is included in the block I'm generating only if it gives me enough bitcoins as a fee. If enough (generating) clients did this, it would be advantageous for you to add the fee to your transactions, so that they are processed faster.