Generates nice object to test quality (how flat is it) of your print bed, its primary for heated print bed if you print without raft (I use custom modified Raftless plugin originaly by Zaggo http://pleasantsoftware.com/developer/3d/2009/12/05/raftless/ ). Flat consistent surface is key for this.

Open in OpenSCAD set your parameters, compile it, ..., export as stl file. Then skeinforge it as regular object (without raft ... otherwise its useless). You should stop printing after outline of first layer is done, printing whole object is waste of time and fillament. After print you can examine the bed by looking how wide is outline wall printed. It should be as consistent as possible.

In parts where the wall is wider is the bed little higher, and if the fillaments just sits on the bed, there is it lower.