Okay, to summarise what was found on that thread:

Princeton Tec Aqua Strobe - 3 Mile (4.8 Km) Visibility
Apeks Torches - Apeks STROBE - 3.4 Km (2 Mile) Visibility
Inova 24/7 - Up to 2 Mile (3.2 Km) Visibility
ACR MS2000(M) - Peak Luminosity 250,000 Lumens
Visibility: Military tested at a distance of 6 miles (9.6km) on a clear dark night

Anyone have any ideas on how this compares to, say, a Nikon SB-600 Speedlight?

If you don't need the omnidirectional aspect of the strobe, I imagine you could get better (directional) results by holding it next to a signal mirror or a piece of space blanket/mylar reflector. (You could switch the strobe from one side to the other every couple of minutes, or even turn slowly in a circle, to get full coverage.) Of course, a camera flash is pretty unidirectional to begin with <img src="/images/graemlins/smile.gif" alt="" />