thing:3202 is awesome, but the belt rubs against the carriage, so thing derivative is an attempt to fix that.

The latest version of the openscad files and STLs is available from http://gitorious.org/hacks/things/trees/master/makerbot-y-stage

Movie of it in action: http://www.youtube.com/watch?v=IdUrudecH2Y

20101004 update: Moved vertical bearings and added more strengthening

See thing:3202 for instructions for now :)

If you have a heated platform you can print this upside-down to get a really smooth and level platform.

To work with the openscad file do

git clone git://gitorious.org/hacks/things.git
cd things
git submodule init
git submodule update

That will get the files in the git repository and check out the awesome MCAD library that is used to draw the bearings.
The openscad file is using some features introduced in openscad 2010.05, so make sure you have that version or newer.