Thing:3202 is amazing, but the belt rubs against the carriage, so thing derivative is an attempt to repair that.
The most recent edition of this openscad files and STLs can be obtained from http://gitorious.org/hacks/things/trees/master/makerbot-y-stage
Picture of it in action: http://www.youtube.com/watch?v=IdUrudecH2Y
20101004 upgrade: Moved vertical bearings and added more strengthening
See thing:3202 for directions for now :)
For those who 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
git submodule init
git submodule upgrade
That will find the files in the git repository and check out the awesome MCAD library that is used to draw on the bearings.
The openscad file is using some features introduced in openscad 2010.05, so ensure you have that version or newer.