Peter Jansen over at the reprap Builders Blog just posted a great article. He has been experimenting with a powder printer using selective laser sintering and has just published his design for a very inexpensive powder printer chassis. At this point it only has the two bins and the z-axis control hardware but it is a great start. His design could well become the de-facto standard for an open source powder printer. you would only have to figure out how the x and y hardware works and, if you are using a salvaged printer mechanism, only one of those.
The open source powder printer chassis
There is no scale in his article but I believe that the build area is quite small. He is, after all, experimenting so he doesn't want to build a large machine to prove his approach. Scaling up the design, though, should be fairly easy. I would like to see something with a build area that takes advantage of the capabilities of a salvaged printer mechanism. That is, either 8.5 x 11 or 8.5 x 14 inches.
Peter's geared drive mechanism for the bins
He uses a sqeegee rather than a roller to spread the layers of powder. I am not enough of an engineer to know whether this will work with all materials or not but is seems to me that it will have higher drag and might decrease the life of the motor.
His design has been posted at Thingiverse so we can all start right in on replicating and modifying it.
By the time I get Mendel working and start looking at powder printer construction I expect that this design will have have most of the bugs worked out. Since laser cutting is getting fairly cheap and very available I think that it should come together quickly except for the software.