This is a retractable Z-Probe.
With a Z-Probe your Makerbot is able to "auto home" not only X & Y but also the Z axis. This is done by mounting the endstop on your Makerbot in a way it senses the build platform directly (and nearby the real nozzle).
In order to get out of the way during printing, the sense-pin is retractable.

Although this is a working thing, especially the software changes are still beta. Both, firmware and ReplicatorG/host software, need to be updated. The extended versions are available as source code on GitHub.

For more information (and a sample video), see
http://pleasantsoftware.com/developer/3d/2010/09/14/finding-home/

The design is for use with a Futaba servo S148. That's the servo I found in my junk box. I guess each servo has a different size. So the design might be more a draft for your own design. I plan to try the same thing with a smaller (and cheaper) "micro servo" in the future. I hope I'll find the time to create a parametric OpenSCAD design until then.

Since it's still a prototype, the servo is currently glued to the base plate with some hot glue.

Both, the new extruder firmware as well as the extended ReplicatorG host software, are available on GitHub. I forked both projects from Makerbot:

G3Firmware: http://github.com/zaggo/G3Firmware
ReplicatorG: http://github.com/zaggo/ReplicatorG (ZProbe branch)

This is all still in a very early stage, both hardware and software.