Let me jump in here…Mike did a good job but I want to cover it a bit more.
What they did on the servo you pointed out is use one wire for incoming and outgoing data. This works great but then you can only send or receive (half duplex).
All that is easy with the exception of one problem…how would you connect 2 wires from FEZ to one wire on servo? (you should make a wiki page on our project website once this is working)
You can implement the circuit they pointed out, which uses 2 different logic gates. You can actually do this with only 74HC126 but then you need 2 pins from FEZ, one to enable TX and one to enable RX.
Use a general purpose transistor to reconfigure the TX pin for UART as open drain. Leave RX always connected. I do not recommend this if you do not know what I am talking about.
This can be done in software I think but it requires you to use register access to control the processor directly. This is not easy for beginners. I personally do not mind making code driver for it if you send me one of those servos as a gift