The problem you (Humanoido) will run into is noise. Although you may have 40 pcb's with props, the length of wire that you need to cross connect all the props is such that, you will need to modify your program to use the onboard crystals, doing this gets the props misaligned from seperate crystals, this would then require a communication scheme using an ack reply/line. Using 40 props on a breadboard is very difficult, the program has to be perfect, so does the circuit. Getting 40 seperate pcb's to run like this would be nearly impossible, due to the noise. IF it were possible, it would involve some kind of point to point, as short as possible connection scheme, which was soldered into place.
If you're really serious about 40 props working together, do it on a single pcb. You could make a pcb with 40 - DIP40 sockets in it. Each DIP40 could trace out to "via" soldering points, and continue on to connect other props. Make all traces so you can cut them at a certain point if needed. Between two "vias", this would allow you to re-connect them if you want. IF you use a slower clock that gets passed to the slaves, you might be able to get them all to work like this, on a single clock source. However you don't have much room to use other clock speeds. After 10-20 pcbs i think your clock would look too much a mess for props to like.
Clock Loop, thank you for the good suggestions. The plan is to use 20 Parallax Proto Boards for a total of 20 props. Boards will have the extra prop chips removed, and a very close spacing - as little as .75-inch apart to minimize noise. The master will use its eeprom to boot. Slaves have no crystal. As you say, the clock can be adjusted. The slave eeproms are 64K and can be used for the upper 32K. The 1st 32K will have the placebo program so as not to interfere during booting. The software is what you posted. Will try this as soon as I get some time.
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http://forums.parallax.com/showthread.php?127983-55-Parallax-Propeller-s-Parallells-Processing-of-Permanent-Perturbations.