I've seen transaction fees mentioned a few times but I don't see any way in the client to implement them.
Looks like they will be implemented only after emission of 21 million BTCs will be close to an end.
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.
Right. I was asking if this is going to be an option that will be a part of the GUI for the bitcoin client. Making it simple for someone to charge transaction fees if they so desire.