UPDATES:

New video showing awesome Z speed:
http://www.youtube.com/watch?v=87sbzmQVkww
Thanks to Jetty's Accelerated Firmware: http://www.thingiverse.com/thing:15380

• The original 0v0 zip did not have all the needed files in it.
  The current MiZ_Rider_0v0_fixed.zip corrects this.

• There is now 0v1 uploaded. This corrects some fiddly stuff I
  found when I assembled the thing. MiZ_Rider_0v1_No_STL.zip
  has this latest OpenSCAD code but I have not had time to generate
  the STL.
  Misery_Z_Rider_0v1_STL_ONLY.zip
  has the 8 STL build plates, ready to print.

MiZ is a complete Z-Stage replacement for the MakerBot Cupcake. Almost all the parts can be printed on your MakerBot. It will work with many extruders: MK5, MK6, MiseryPusher ( http://www.thingiverse.com/thing:7129 shown) as well as any other extruder that is designed to attach to the standard MBI MK5 style extruder base plate (such as: http://www.thingiverse.com/thing:5684 http://www.thingiverse.com/thing:6713 http://www.thingiverse.com/thing:7113 http://www.thingiverse.com/thing:7037 ).

The goals were to have very few modifications to the MakerBot, and to have a rigid structure that would yield good quality prints (like:
http://www.thingiverse.com/derivative:8582 )

With my setup I can get 550mm/minute Z-travel rate:
http://www.youtube.com/watch?v=K-9MwgUz1v0

Here is a longer and more boring video:
http://www.youtube.com/watch?v=Fuudemoq2MQ

And yet another showing stability during a high-vibration fill:
http://www.youtube.com/watch?v=_Z8G90gfEYI

Excluding nuts & bolts, re-used parts, and shipping the non-printed cost is as low as $26:

• 2 of 12mm x 13 rods: $5 each
• 8 of 623 bearings : $10/10 (shielded) or $20/10 (sealed)
• ABS glue: $6
  Links to the parts are in the instructions PDF.

I do have some tweaks and improvements in mind. Please watch for updates.

Heritage -- This design owes a lot of inspiration to these two things:

Z-Rider by twotimes http://www.thingiverse.com/thing:4740

Type A Z-Rider. v3.2 by TheRuttmeister http://www.thingiverse.com/thing:5586

Suggestions:

whosawhatsis recommended his "Minimalistic X-rod mount":
http://www.thingiverse.com/thing:6884
instead of the version in the file. His is a nice design and should work well, it is lower profile and easier to open/close.

If you do not like waiting for Skeinforge, using your SD card for builds, going to plumbing section of the hardware store, cleaning up parts, gluing, clamping, and generally doing mechanical fiddling, this build may not be for you. But if you did not like stuff like that, why would you have a MakerBot?

There is a 15-page how to build it PDF. Please use "right-click, Save As" get it on your local disk for viewing.

The ZIP file contains the OpenSCAD files and the eight STL files need to build it. I used high resolution curves, which is overkill and will make the skeinforge process really slow. Maybe start it and go to lunch or bed or something. You will want to build from the SD card since there will be a lot of resolution in the curves.