The video threat against the NYSE included a call for Anonymous supporters to utilize the so-called Low Orbit Ion Cannon (LOIC) voluntary botnet tool to launch a Distributed Denial-of-Service attack against the NYSE's website on the target date.
Regardless of whether this is a threat from Anonymous or a fake, I really don't think DDOSing the web server(s) of the NYSE would have any affect whatsoever on their actual trading systems. I would assume (or hope) that there is no internal connection between the two.