Better solution to a very old problem!
Plug a mic into one of your interface channels, set up a track in GarageBand with that mic set as the input (make sure monitoring is turned on on the track), and whenever you need to use the talkback mic, just arm the talkback track for recording and you're good to go!
This is a very old thread, but if you google "Talkback mic in GarageBand", this is the first hit, so an updated answer may help people like me.
Since this answer was written, Mac OS has received two incredibly useful features in Audio Midi Setup, called "Aggregate Device" and "Multi-Output Device"
For this answer, we will need an Aggregate Device. This allows you to take all of your audio devices (USB interfaces, Firewire interfaces, built-in devices, both the headphones in/out, as well as onboard mic and speakers) and put them into a single virtual audio interface.
The great advantage of this is that, in GarageBand, you no longer have to pick just one of the interfaces for your inputs. Now you can pick this aggregate, virtual device, and all of your inputs are going to be available on every track.
So, for talkback mic, you don't have to sacrifice any of your interface inputs. Just pick your built-in mic on your Mac(Book), and assign it to your talkback track (as per the original advice above).
The other feature, Multi-Output Device is great for monitoring while recording someone else. You create a new Multi-Output Device, then assign every interface you have for simultaneous audio output. Choose the new multi-output as your output interface in the GarageBand and you're good. That way, whatever is coming out of your GarageBand will be sent to all of your outputs simultaneously.
Now, you plug in your own headphones into your Mac headphone output, you give the other set to the person you're recording, plug them into the interface (for direct monitoring),. They set their own monitoring volume on the interface, you set yours on the Mac. No need for a multi-headphone amp.