A while back I picked up some ultrasonic proximity detectors from BG Micro (they don't seem to have them any more). They appear to be called "Mini-S Sensors", and have one output that detects at 7 feet, and one that detects at 5 feet.
More info on them is here.
Blame progress is coming around nicely. I have a nice steel superstructure now. I picked up some 4066 Quad Switches to control the button switches MP3 player (the strategy appears to work, hurrah!). And am painting the arm of Blame to get a nice image for the gallery postcard.