Blender artists thread:
ZIP that includes the script (Blender 2.5), PDF help file and blend file with the default brush (for spheres), and the custom brush shown in this video. Also the matcap and material used in this demo.
Run first the script ArraySketch.py, and then ArraySketch_hotkeys.py to register the hotkeys.
(Note that this script is on an alpha stage. I've tested it with SVN revision 26022, and may probably need changes for future Blender releases).
Update: As for SVN 26510, everything seems to work fine.
Update May-7: A video-tutorial was kindly made by Dimetrii . The voice is in Russian, but it clearly demonstrates the features of this tool, and it has some subtitles in english:
Update April-9: This is a Blender version I used that worked fine with this script...
A grease pencil video tutorial that may help get a grasp of the actual grease pencil features: