how about something using a similar principle to bubble jet printing technology?
Based on what I read here:http://en.wikipedia.org/wiki/Inkjet_printer
I don't know whether such technology could be adapted to a continuous circulation fountain (which is what one of these displays ultimately are), or whether such technology could be fabricated at a larger scale by a hobbyist, or whether such technology would even work at larger scales.
Furthermore, since inkjet technologies rely on propelling droplets, whereas in the videos of the waterfall displays, you can see only gravity is being used (plus there's the fact that if the droplet nozzles were pressurized, the change in pressure from opening one or more would have a cascade effect on the amount of pressure behind others falling - which may or may not be difficult to compensate for in software).
But the idea is interesting - right now, no matter what, the goal to achieve this at a "homebrew" level is getting the cost of the valves to a low price per valve; such a system would need at minimum 5 valves to make characters (5x7), but more valves would be wanted to make the kind of effects shown in the videos (probably 64 at least on the low end, 100+ on the high end). If a valve cost $5.00 to make, such a fountain/display would get expensive quickly (though still much cheaper than the shown fountains, most likely).
I think my idea, if it worked, would bring the cost down to less than $5.00 each, possible much less (maybe even $1.00 each). But all options and ideas are worth trying and investigating, because they might lead to cheaper options.
One possibility might be to instead replace the ball bearings I noted with perhaps short lengths of a nail of similar diameter, with the tip of the nail ground or filed to a rounded point; that would entail more work, but might be cheaper than the ball bearings (though the steel bearings from smallparts.com are pretty cheap already).