I'm surprised that it has taken this long. The vast majority of stock trades are currently performed or mediated by programmatic trading, so it only seems fitting that the signaling mechanism also be computer-driven, whether it involves a physical actuator and sensor or just a timer tick. On the face of it this is just a publicity stunt, but there have been a series of stories over the last few months about advance access to information that really highlight how ubiquitous automation is becoming in stock exchanges and how some people are taking advantage of it. The term 'day trading' has become quaint: Most of the action is at the millisecond level these days.
The ironic thing about the Universal Robots' arm ringing the NASDAQ bell is that it is not included in the ROBO index! In fact, most of the coolest robot makers are not listed because they are private companies--yet to make an intial publich offereing (IPO). However, all the big name public robot makers are on the list, such as Fanuc, Kuka and iRobot.