This is a very simple, but helpful OpenSCAD script that can be used to generate three types of printing plates based on 2D DXF designs.

Essentially, this script extrudes a 2D design (loaded from an external DXF file of your choice) and either adds or subtracts it from a flat plane to give an effect not unlike relief or intaglio work. You can take these plates and use them for embossings, debossings, stenciling and possibly even ink printing!

I created this script to make it easier for undergraduate printmaking students at my university to explore 3D printing in their work, and to gain some experience with digital fabircation.

You can see more plates and prints made using this script at: http://www.flickr.com/photos/zenwebb/collections/72157631741058233/

NOTE:
I am not an experienced printmaker, so I may not have gotten all of the terms correct here!

Complete instructions with examples
I've created a lesson plan on my website including tons of info about printing plates using this script and producing prints. It contains the same basic info as below, but with more pictures and explanations!
http://jason-webb.info/2013/03/how-to-make-and-use-3d-printed-relief-intaglio-and-stencil-plates-for-printmaking/

Prepare a 2D DXF design suitable for OpenSCAD
To utilize your 2D design, OpenSCAD requires that it strictly adheres to the R12 DXF format. In my experience, this means only non-intersecting polygons made up of only straight line segments. All effects such as fill and stroke are ignored, only the geometry (paths) is utilized.

I used Inkscape to draw my designs because it's free and natively supports R12 DXF exporting. Follow the procedure outlined in the following link to get up and running: http://repraprip.blogspot.com/2011/05/inkscape-to-openscad-dxf-tutorial.html

Design considerations
Depending on your application, you may find that small features will tend to break under the pressure used to produce prints, especially embossings. You can try altering the design to have thicker features, changing the design_thickness parameter to extrude the design less, or apply less pressure with your pressing technique.

When making stencils, you'll want to be careful that all features are properly connected and well supported, otherwise you'll end up having features bending or breaking during printing.

Using the script

1. Download the .scad file and open it in OpenSCAD
2. Create a suitable DXF design in Inkscape (see above), and update the "design_filename" parameter in the script to point to it. Be sure your document dimensions match the dimensions of the plate in the script.
3. Call the name of the module you want to run in OpenSCAD (more notes in the script).
4. Hit F5 (Compile) to quickly check that things at scaled and positioned right.
5. Hit F6 (Compile and Render) when you're ready to get an STL file of the plate.
6. Use "Design > Export as STL" to save the plate somewhere.

Slicing your model

1. Raft - your choice. I printed mine with a raft, but if I were able to print without one it might be easier to bond the plates to another material.
2. Infill - bump it up to 100%. The plate will be thin, so you will need all the strength you can get to have the plate stand up to the embossing/debossing process.
3. Layer height - doesn't make much of an impact on the plate or resulting paper prints. Keep it around 0.2mm-0.3mm to speed up printing time.
4. Shells - make sure to use zero shells! This will let Skeinforge fill all of your small details with filament and minimize ugly gaps and defects in sharp features.

Producing embossings, debossings and prints
I worked with a printmaking professor at my university to actually produce prints, so if you are looking for a clear step-by-step explanation of how to produce prints, you may want to seek out an expert to work with. The vocabulary and interpretations of printmaking techniques can be different depending on who you talk to, so the important thing is to make your plates first and then find someone to help with the application.

Notes about ink printing
Compared to traditional media, ABS/PLA plastic is relatively porous and highly textured, so ink tends to get trapped on these plates very easily. Luckily, this process is relatively cheap, so print multiple plates to do multiple types of prints! You can also try using a sealing agent like polyurethane or shellac to prevent the ink from seeping in.

Notes about material properties
Compared to other media used in printing, ABS/PLA is very weak, so if your printing press applies too much pressure you can easily destroy the plate. Working with a printmaking expert would be a very good idea here, as they can help you set the press correctly.