The palm-drawing poll on this forum missed ZiPCAD! While it is not a "pixel"-paint program, it's just the best solution for vector based to-scale drawing on PDAs (I mean PDAs not Palm OS devices only) !
It can read/write native DXF files from expansion cards (and boy, it's FAST) and is FASTER than any PocketPC CAD solution! I bought it and could not be more pleased with the software! Coordinate input solution and patented walls&gaps tool is again the FASTEST way to vector-draw (for me, it beats even desktop AutoCAD keyboard/mouse combo)! Try the demo and say something : http://www.zipcad.com